It's true you will get traffic which will help your site to rank well on search engines, but you can also connect with people who can working with future projects together
You will get TARGETED traffic - this is really important. You should not want any traffic, you need "good" traffic that will be your customer. I don't think it will help in your page rank though
With facebook, what I would be after is traffic. It had been my expectation with this social network when posting in groups.
You will get a lot of things: 1) Traffic 2) Backlinks, when people start talking about your site 3) Page rank, because of those backlinks.