Social network is good for generating web traffic, There are many social networking sites like Facebook, Google+, Pinterest, Twitter, StumbleUpon etc. Each have its own importance in SEO. I think Facebook, Twitter and G+ are great social media sites for SEO. These social media networks have millions of users that can help you further promote your site. Not only do you get a backlink on these social sites, you get traffic too. Personally, I use Google+ more rather than other.
It all depends on what you are trying to achieve. I appreciate the durability of Google Plus, the visual appeal of community that Facebook and the up to the minute updating of Twitter.
I think site owners and admins tend to spend so much time developing a social profile for each, trying to maintain each and find new ways to promote their site that they lose interest in their own ventures sometimes.
SEO is such a delicate, yet required aspect of gaining a presence online. I realize there are ways you can optimize each social media outlet to generate interest with a new audience, influence pride with current members and thrown in relevant keywords but it's .... a headache sometimes across each platform.
I try to stay minimal with my posts on all the outlets I set up for my company. I provide services to my web design clients with emphasis on my social media expertise, when I offer this option I generally register them with a limited number of social media sites with one platform allowing them to do a blanket post on all outlets. Again, it's a very delicate dance when it comes to ensuring the posts are going to remain informative, associated with the general ethics of your business and relevant.