One of the problems with social networking sites is that they tend to be ugly. Not just a little bit ugly. Seriously ugly.
MySpace has upgraded its UI, but it's still noisy, long (you have to scroll way down on some pages to get where you want to go), and cluttered. Facebook? It looks like a dingy and, like MySpace, is overly noisy (It's also shockingly slow. I've been trying to change my account settings and it keeps hanging on me). Not only that, but my home page on Facebook doesn't seem to want to let me cut out all the noise - no matter how much I tell it that I don't want to hear about so-and-so "friend"ing so-and-so, it keeps telling me.
Only LinkedIn seems to respect me as a grown-up. It lets me filter out noise and presents everything in an inviting UI.
The other thing I like about LinkedIn is that it doesn't try to be my one-stop shop for social activities. I use it for one thing: Professional networking (and recruiting as a fortunate off-shoot of that). I would never dream of paying Facebook or MySpace to blare fake friend activities to me, but I gladly paid LinkedIn $500 to help me recruit a few people to my company each year.
I suspect the noisy design of Facebook and MySpace has more to do with wild attempts to be relevant to more and more people (so that they'll visit the site and get hit by advertising). I prefer LinkedIn's more direct, conservative approach. I visit it when I need it, and I pay for that "need." Form follows function.