I've added social bookmarking buttons/links on my blog (not very active) and forum and very few people actually used them ... (my audience is not so web2.0)
It still bring me a small amount of extra traffic each month which is fine with the time I spend on that (nearly none).
Getting a few backlinks from social media sites can't hurt and is good to get a wider variety of inbound links.
I believe that for traffic it could be a good tool if you can spend time but for link building I suppose it doesn't work much and as RedCardinal said, Google will more than likely take less and less into account those kind of links. I'm sure some do-follow social bookmarking sites will go nofollow at some stage to avoid being spammed.
As many other tricks in seo : do it with moderation
It still bring me a small amount of extra traffic each month which is fine with the time I spend on that (nearly none).
Getting a few backlinks from social media sites can't hurt and is good to get a wider variety of inbound links.
I believe that for traffic it could be a good tool if you can spend time but for link building I suppose it doesn't work much and as RedCardinal said, Google will more than likely take less and less into account those kind of links. I'm sure some do-follow social bookmarking sites will go nofollow at some stage to avoid being spammed.
As many other tricks in seo : do it with moderation