Research: what makes Facebook so engaging?

I'm writing a talk for the Enterprise conference at CeBIT, which tries to unpack what makes people spend hours and hours a week interacting with and updating Facebook, and what enterprise applications - particularly those that benefit from collaboration and which are often "higher level" tools rather than lower level tools necessary for a narrow job function - can learn from them.

I'll be sure to share my thesis for what companies and enterprise applications can take away from Facebook - and the things they should make sure they leave behind - back here on my blog, but until then I'd really love to hear from you: what do you think makes Facebook so very compelling that keeps users "hooked" on it? Please share your thoughts in the comments!