You may never know why. The email will arrive and reasons offered for your rejection might be the real ones—or not. Companies do not always like to admit that they have been scrolling through your information posted on your social media sites. Yet many, particularly the big ones, are doing just that. And in increasing numbers, they are acting on what they find when deciding whom to hire and whom to pass over.

Even before the pandemic, online checks on applicants were common. Research in 2017 found that 28% of large companies had turned down applicants after carrying out such checks. Reed Screening, a large agency doing this work, was asked to run over a third more social-media checks in December 2021 than a year earlier. And in 2022, the UK government gave the practice approval when the Department for Education said schools and colleges “should consider carrying out an online search (including social media) as part of their due diligence (尽职调查)”.

The way screening works is simple. Any “problematic” behaviour will be marked with a red flag. There are laws governing such online screening. David Erdos, Co-Director of the Centre for Intellectual Property and Information Law at the University of Cambridge, says that companies are supposed to warn candidates before screening their social media and tell them that they have the right to refuse consent (同意). Some companies follow these rules, but others may not. As for the right to refuse, it is hardly practicable. “Who on earth is going to be that person who says ‘Hang on a minute — I’m not happy about that’?” says Mr Erdos. “That itself is likely to be a red flag.”

All agree that the spread of online screening is alarming, as it is so unforgiving. People may be unable ever to be free of their online past, says Mr Erdos. Some footballers recently were punished for comments made almost a decade ago when they were but teenagers, as they have found to their cost.

Those within the industry think laws should be tightened, with punishment for illegal acts. In the meantime, caution (谨慎) is advisable. In talks with university students, Keith Rosser, director of Reed Screening warns them what companies will do to their social media when they apply for jobs. They are “universally terrified”, he says. And rightly.

Donald Trump has been known to do it in the middle of the night. Kim Kardashian has done it more than 22,000 times. Many people don't understand why you would want to do it at all.

We’re talking about tweeting and, unlike posting your entire life on Facebook, this social media activity demands a particular set of skills.

Why use Twitter? It’s a way to have a one-on-one virtual connection with your heroes and anybody who interests you. Likewise, you can share your life and views with an almost limitless number of people. And it all has to be done within the limit of 140 characters of text – plus photos, videos and links – which really helps you focus your mind.

Katy Perry has the most Twitter followers, with 95.6 million of the micro-blogging site’s 317 million monthly active users. Justin Bieber isn’t far behind with 91.5 million, followed by Barack Obama and Taylor Swift with around 83 million each. The heaviest hitters of Twitter are pop stars, heroes of sport and other celebrities. But you don’t need to be famous to create a buzz with the right tweets at the right time.

In fact, Katy Perry is a great example of how to do it well. She has a bubbly tweeting style that mashes up chat about her tours and her music, insights into her emotional roller coaster over the US election, and playful descriptions about her daily life. "Done with my Christmas shopping", she tweeted, and posted a link to her Instagram page that featured crazy gadgets like a "pet emergency jacket" and a "chocolate donut camera".

To get started on Twitter, pick a simple, memorable user name like @KatyPerry, post a short profile and choose a photo. Some tips: post your own photo (not a photo of your dog, it’s not Facebook) and don’t be an “egg person” – referring to the absent egg-shaped image when a user hasn’t posted their own picture. In that case, Twitter won’t get you any followers or respect.

Then, it’s time to start tweeting by sending your updates (“what’s happening?”) and following people. Building up an army of followers can take time. Generally, the more people you follow and the more you tweet, the more followers you’ll get. To become a Twitter star, you need to carve out a reputation for posting original, eye-catching tweets on trending topics – and maximise your visibility by mastering the use of hashtags.

Twitter Analytics will show you how your tweets are performing and who is following you – right down to their age category, country, income bracket, gender and interests. If your popularity is flagging and your Twitter ego is keeping you awake at night, you can turn to the business of buying thousands of "followers" from online sites. But do be ware – there may be a lot of fake profiles in there and, at the end of the day, it’ll feel like paying a crowd of people you don’t know to come to your birthday party.

Happy tweeting!

What may well be the oldest metal coins in the world have been identified at an ancient abandoned city known as Guanzhuang in China. Like many Bronze Age (青铜时代) coins from the region, they were cast in the shape of spades (铲) with finely carved handles. These ancient coins existed during an in-between period between barter (以物易物) and money, when coins were a novel concept, but everybody knew that agricultural tools were valuable.

Reading about this incredible discovery, I kept thinking about the way modern people represent computer networks by describing machines as having “addresses”, like a house. We also talk about one computer using a “port” to send information to another computer, as if the data were a floating boat with destination. It’s as if we are in the Bronze Age of information technology, grasping desperately for real-world reference to transform our civilization.

Now consider what happened to spade coins. Over centuries. metalworkers made these coins into more abstract shapes. Some became almost human figures. Others’ handles were reduced to small half-circles. As spade coins grew more abstract. people carved them with number values and the locations where they were made. They became more like modern coins, flat and covered in writing. Looking at one of these later pieces, you would have no idea that they were once intended to look like a spade.

This makes me wonder if we will develop an entirely new set of symbols that allow us to interact with our digital information more smoothly.

Taking spade coins as our guide, we can guess that far-future computer networks will no longer contain any recognizable references to houses. But they still might bring some of the ideas we associate with home to our mind. In fact, computer networks — if they still exist at all — are likely to be almost the indispensable part of our houses and cities, their sensors inset (嵌入) with walls and roads. Our network addresses might actually be the same as our street addresses. If climate change leads to floods, our mobile devices might look more like boats than phones, assisting us to land.

My point is that the metaphors (比喻) of the information age aren’t random. Mobile devices do offer us comfort after a long day at work. In some sense, our desire to settle on the shores of data lakes could change the way we understand home, as well as how we build computers. So as we cast our minds forward, we have to think about what new abstractions will go along with our information technology. Perhaps the one thing we count on is that humans will still appreciate the comforts of home.