If I were LinkedIn, I'd sell. Facebook is going to crush them.
Until Facebook makes some significant business-friendly changes to their site, I think LinkedIn still has plenty of room to exist and/or grow.
Where's the intersection? I like linkedin. I've had a few recruiters and business folks contact me through there. I'd be very reluctant to put my career stuff on fb. Perhaps some people mix business and pleasure?
Granted I'm still in college, but my Facebook profile and my LinkedIn profile are like night and day. I do put some of the information from LinkedIn on Facebook (they do have a Work & Education section in profiles), but that's not why people look at my profile.
I certainly wouldn't want the comments on my wall showing up when people are analyzing my profile as a job candidate.
The two services just don't mesh very well on a high level.
It seems to be working pretty well for a lot of people.
Basically, I think it makes the wrong assumptions about the way people network.