I recently received two invites from someone I know, one for Facebook and another for LinkedIN. I accepted the Facebook invite right away, took me over 2weeks to activate the LinkedIN connection. The difference was that I could add the contact to Facebook via my cellphone but not so with LinkedIN. For the latter, I had to go to the site, log in and accept the invite. With Facebook, I can add a new contact with a simple text message.
For sites trying to acquire new users, this is a lesson worth learning, the easier you make it for folks to sign up, the faster you can drive adoption.