I have a plugin that updates Facebook and Twitter. There are other networks available but I don't normally use those, so I left them out. Yes, those updates do bring in traffic. I have a plugin which tells me where the visitors are referred from and there are some who come from the post on my Facebook page.
I too, have a plugin that updates Facebook and Twitter automatically as I post messages on my blog. I also have a plugin that gets updated on my blog whenever new friends are added to my Facebook or Twitter account. This allows my users from both sites to be aware of my blog - I find that this drives traffic to my website.