i run
http://twitterbabes.allpornmodels.com/ (where people can "vote" for pornstars on twitter, even though it has no point to it)
from the voting i have on there, it seems (rough guesses from looking at the db entries, no real stats worked out ) 80% are old men who repeatedly reply to pornstars tweets (in the hope of getting a reply??), they will repeatedly vote for certain pornstars (raven riley seems to have the biggest following ever. some of them even email me asking about the votes and stuff, and if the site is down for 10 mins ill get emails or messages on twitter asking about it because they can't vote for her).
those guys do seem like the type that join sites though. but they seem to fall in love with one girl and just @message them all the time. i think its worth it if you're a sologirl/babe, but for a general babe site i dont think its worth the effort.
plus i recon one day twitter will crack down on adult links. you can't have boobs etc in the background / avatar pics at the moment (but it seems linking to porn is ok)