Tech publisher extraordinaire Tim O'Reilly handles a tsunami of social network friend invitations on a daily basis and says he accepts the ones that actually explain who the initiator is:
Most of these [friend requests] , relying solely on the boilerplate invitation text, go right into the trash. "I'd like to add you to my professional network on LinkedIn." Sure. Who are you? What do you do? Why should I care? (Even if I've met you, I might need my mind jogged, especially if you might have the same name as other people I know.)
The art of the well-written social network invite is similar to the art of asking questions of someone who doesn't know you: give 'em a reason to answer (or friend) you.