What killed Netscape is that version 3 of their server was a dog, and everyone switched to Apache or Zeus. I was there.
My employer at the time spent 6-figures at a time on Netscape server products, for ourselves and our clients, we must have spent millions with them. Then version 3 came out and we either stuck to 2 or moved off Netscape platform entirely. And that was a big deal, since we had written loads of NSAPI code! But that's what killed Netscape.
I think Orkut provided a strong datapoint for your question. They supported a professional and personal profile when they launched, and the overall finding was that a single site supporting social + professional networks is a turn-off because most people want to keep those networks separate (since many of your friends have no interest in your resume, and since you don't want your boss to see your pictures at some party).
One potential issues is that there are really 3 groups of people: friends, coworkers, and "both". For each posting, photo, action, etc, you want one of the three groups to see it. That becomes a hassle compared to just keeping friends and coworkers in separate social networks (one pitfall, though, is that using two social networks makes it hard to send messages to friends and coworkers without having to write the message twice).
That said, Facebook has a ton of smart people, so maybe that can figure out something that Google could not.
One last thing: I think part of the perceived value of LinkedIn is that they have not deviated significantly from their goal of being The professional network. I remember it took them a long time to even add profile photos, because there was a debate about whether those were more "professional" or "fun". I think it would be as hard for FB to become a professional network as it would for LinkedIn to become a social network (while not losing its professional member base)
So while the userbase is certainly very large, the revenue potential is very limited outside of those core activities.
What does it mean to be firmly on "maybe"?
To me, that's a clear sign of solutions looking for problems. In other words, "there's gold in them hills".