This is very true. I can't bear the deafening sound of trumpets blowing on FB so rarely visit it. On occasions though, it's service as a 'Friends Reunited v2' works well for me.
Isn't that one of the main features of LinkedIn? Maybe I'm misunderstanding but aren't you essentially saying the equivalent of "I don't use Google's services but it's a handy way to help me find websites"?
Somebody "friends" me on LinkedIn, I accept. If I need to contact that person and I don't have their email, I go into LinkedIn and send them a message.
So it's essentially a backup in case my address book fails. I'll pop in there maybe a dozen times a year. That's not exactly using their service.