Just read thread on Webmasterworld, which started with post asking if links from blogs and social sites are useful. Posted this:
To maybe annoy Coburn [previous poster; had just given figures showing Digg leading to massive traffic surges, with good sustained results], as no scientific results, yet keep short:
Seems to me that much of this thread is about putting crappy links on crappy sites/blogs.
No statistics from me, but I've certainly found strong blogs - including on science (global widgeting, say). Made occasional comments on these, with links to sections of my sites that are relevant. Only occasional, so no stats, but surely a good move - and over time, can lead to getting noticed if some knowledgeable bods follow the links, find I have good content. Previously seen traffic/links resulting from participating in discussions on the much feared avian widget lurgy.
So, even in "2.0", surely almost the same old same old: if you're gonna try slapping links any place, can of course expect each one to be near worthless, and to get deleted darn fast if site owner doing his/her job. So, find some folk have automated the posting task, and can inundate forums/blogs if find flaw in software (yeah, I've experienced this: 200+ crap posts within 10mins; made changes since).
While, if you find places within your zone of expertise (you do have some, don't you? - you're not just writing unoriginal woeful fluff?), and add comment/link that makes contribution, links from blogs and social sites can surely help.