Using heuristics to look for behavior patterns is a good idea. Implementing dumb heuristics just ticks off your users.
Case in point: Facebook. I rarely have need to use it much, but recently I had the opportunity to make a couple groups, and sent some invites to folks to join the groups. Well, Facebook decides to tell me that I’m using it to spam people and they’ll deactivate my account if I continue. Fine, I sent an e-mail to the folks instead (aside: an ad-driven site telling its users to stop generating more page loads?!?!) which was actually much easier.
Then I went looking for a couple folks I’d lost touch with and sent them friend requests – and it still told me I was using the site for spam. All I can figure is that friend requests must internally generate messages and it tripped on the same shortbus algorithm.
Googling for this leads to kinds of people complaining about the same thing – some even are banned for life for sending ‘too many’ friend requests. Facebook CSR’s apparently have authority to do so. No wonder Microsoft bought these guys.
I also setup an Orkut account. That I just had to go look up how to spell Orkut is one of the many things that’s keeping Google from squishing Facebook.