It's a good question, and one that clients are desperate to know the answer to. Everyone wants to harness the power of 'influencers'.
So come on, who are they? Celebrities?
By their very nature, celebrities have fans, so Twitter is an ideal channel for slebs to talk about themselves and for fans to listen in. Aston Kutcher has 5 million followers - does that make him influential? Yes, but not as influential as the 0.01% that he follows back. Possibly.
All I'm saying is that influence isn't necessarily just to do with the number of followers. They could all be spambots and inactives. So what is it to do with then?
Tons of real followers? But maybe they're busy listening to other people, not you.
Tons of tweets? They could all be self-promotional, repetitive, auto-generated ... or all three
Lots of interaction with tweeps? Great, but maybe it's just day to day chit chat with a dozen or so buddies.
Lots of re-tweets? Again, that's community-spirited, but are they all the obvious tweets from @mashable that everyone has seen anyway?
Is it to do with how many followers a person's followers have? Now we're getting closer to it. A person with 2,000 followers of which 40% are inactive or bots, 40% tweeters with fewer than 100 followers and 20% friends, family and colleagues probably has a much smaller reach than say someone with 200 followers of which 50% have themselves more than 2,000 followers. You only have to start drawing a diagram of this to see how it works.
One reason why I like the tool Twinfluence is because it looks at not only the size of a Tweeter's network (1st and 2nd degree contacts) but also takes into account factors such as centralisation - the relative dependency of the network on just a few nodes to maintain the connectivity. For example, if you have the Twitter ear of Aston Kutcher and he retweets you, it doesn't matter if you only have a dozen followers (although you won't for long!), you are connected to a huge network.
There are quite a few tools now which reputedly measure Twitter influence, such as Klout (which I have used but find the results a bit erratic) and TwitterAnalyser. They all present a plethora of stats - reach, connectivity, amplification, velocity, frequency, retweets, subjects tweeted about ... but of course the data needs interpretation and my feeling is it's worth consulting a number of sources to get an all round picture. Plus, much of it is theoretical - for example, I happen to be one of the 3% of @stephenfry's followers who he follows back, but since he has never sent me an @ reply, let alone retweeted me (boohoo!), having him in my network makes no difference to my actual influence level. (By the way, a good tool for finding out if a particular person follows you is DoesFollow.)
I'll leave you with a final thought - a great point made by @mediaczar at the recent Social Media Marketing Conference in London - influencers are contextual. They are not set in stone. Their actual influence depends on a range of ever-shifting circumstances. So finding and harnessing the power of 'key influencers' is really like hitting a moving target.
