If you are a Person on Twitter, you probably get emails like this with some regularity, informing you that some aggregated group of other humans and robots have found you interesting enough to follow. "You have new followers on Twitter!" Congratulations.
But sometimes, a Person Of Interest follows you — and you get a special email all about him or her or it, one that breaks them out of the pack of generic followers: "[Person] is now following you on Twitter!" a return to the older style of Twitter follower emails that let you know about each and every person, one at a time. (Ugh, old Twitter.)
The first time I noticed the "special" follower email, it seemed that it was specifically letting me know when people who had a lot of followers followed me. Like (internet) famous people don't deserve to be clumped in with a bunch of rando sexbots that are targeting your account. But it's actually more subtle than that. I blocked FWD's John Herrman as part of an unfunny joke, which meant we unfollowed each other. I re-followed him, and then when he re-followed me, I got a special email just about him.
Another example: When Quartz's Zach Seward, who doesn't have a particularly high follower count, followed me a month or so ago, I got an email that called him out specifically, even though I didn't follow him at the time. The only thing that's immediately visible is that we share a lot of common followers. Twitter's using some variety of signals to determine whether a new follower warrants a special email — whether it think you'd be really interested to know this particular person is following you.
It would seem, on the surface, that the core metric of Twitter is the follower count. But the real metric is interest. (There's a reason I kept using that word.) The most valuable thing that Twitter owns is its interest graph, a map of everything you've shown "interest" in. It is tightly tied to who you're following and who's following you — if I follow Four Barrel Coffee and Heart Roasters — I am probably both interested in coffee. On the other hand, if some baristas follow me, I am probably interesting to people who are interested in coffee. This is a big part of why Twitter cut off both Instagram and Tumblr from accessing its users' friends lists.
But the interest graph is more than that. It's the sum of all of Twitter's data about what's interesting. Twitter cofounder Ev Williams specifically decried the follower count as not very interesting. Rather, he said, "The thing I think would be more interesting than followers is ... retweets." It's a measure of real engagement, of interest.
Interest is something of an invisible, ephemeral metric, but how Twitter sorts your follower emails is one way to grasp it. And obviously, for now, the follower count still rules. Otherwise I've got no way to explain this one: