I put stuff on LI that I never would on FB or Twitter. I'm a complete ghost on those social networks largely because of the need to separate church and state (I am not even my own name on those social networks). And the truth is I get random call-outs for jobs, possible partnerships and introductions, etc., that have been valuable in a business context there. LI probably doesn't make enough $$ off of many of those, which is their problem. I've signed up for FounderDating (dumbest name ever) and the like as well, but they haven't been nearly as valuable for the lack of critical mass as X points out.
You can say that LI has limited revenue potential and that they've never layered on a good business model. But I'm sure I've generated 100x more revenue for LI than I ever have for FB or Twitter.