I'm not quite sure what to make of WeFollow, but it's explosively popular on Twitter. Its like a directory of twitterers, you sign up with your twitter nick and three tags, you can then search for people with similar interests by checking out their tags.
Right now there's about 18,500 different tags in use.*
I've done me a graph of the how many people use each of the top 1,000 tags. Looks like a normal distribution with a logarithmicy axis. There's a bit of a spike for the top twenty most popular tags and the top hundres are bumped up a little from the rest of the curve. I guess its cos they're on the first page of tags when you check out what other tags are popular.
Me, my twitter thing is @illandancient
*you get to page 189 before the number of people using the tags falls to zero