As I’m on all three, I think I can say this safely:
Facebook: For when you’re too smart for MySpace, but too dumb for LinkedIn.
More seriously, I’ve only ever used (as opposed to played with) only two of the web 2.0 properties: LinkedIn and del.icio.us. Of the two, delicious is far more useful. It’s tagging and search capabilities, especially in the del.icio.us for Firefox extension, would be worth it even if there was no “social” aspect at all. I find “popular tags” to be particularly useful, because it saves me those mental clicks in deciding how to categorize a useful page.