A growing number of companies are using information about people’s social networking activity to record their influence online. Put your Twitter handle and Facebook name into a website such as PeerIndex or Klout, and you’ll get your rating. Justin Bieber has a PeerIndex of 89. Stephen Fry’s is 93.

But how exactly is this being calculated? ‘Influence at its lowest level is an outcome of trust that you have within a massive group,’ says Simon Cast, head of products at PeerIndex. ‘You’re looking at the reputation people have in order to work out their influence.’

It all works using complex mathematics. Companies such as PeerIndex take all the pieces of information about engagement within a person’s social networking circles and feed them into algorithms that work out their index score. Importantly, this isn’t just a popularity contest.

If you have a lot of friends and followers, you’re not necessarily influencing their behaviour and tastes. ‘Having followers is the popularity measure,’ says Cast. Instead, services such as PeerIndex take into account your importance in changing behaviour, as well as your reach and the quality of the content shared. If you get re-tweeted by someone, that’s worth something. If you get re-tweeted by someone with a high influence rating, that’s even better.

But it’s not just about getting a high rating. Different people will become influential about certain topics. Klout, for instance, deciphers conversations people have on Twitter to work out what topics they are experts on. If your tweets about lager and pubs get a good reception, then it’s likely you’ll be considered an expert about beer.