I think you're right, Crystal. I believe it's an ease of use thing that facebook, google plus and other social networks have developed meaning that if you are logged in to facebook, when you come here you will automatically be logged in here on your account, if you have linked them. Doesn't seem to work the other way around though... seems the likes of facebook et al don't mind overriding security principals on other sites, but certainly not their own (not saying it's a security issue here at all... just picking nits lol)
It may do other things as well - never really looked too far in to it, personally.
Ahhh, thanks for that. I'm guessing the Facebook icon appears under your screennames because you have entered your FB info into your profile settings, & doesn't actually have anything to do with the "Facebook Connect" function ?