Does social media good for SEO?

I believe Social media is more affect nowadays. And even it become important part to rank....

You can put backlink in forum signature, but this kind of backlinks are not good, small value.

It really depends on other factors, in my experience. Facebook links will do virtually nothing, and Twitter sets mostly all links to nofollow. The only social strategy I found that works at all takes quite a bit of work. You need to essentially create an entire SEO campaign to look like the links are viral, which usually means you need to create numerous wordpress blogs, social media profiles, youtube profiles, blogger pages, etc and then create a funnel effect. This would mean that for example, you want to rank a URL... So you then create a few profiles for sister blogs... Then you write little articles on a similar topic on those blogs around your chosen niche. You then place some deeplinks from those articles into your main URL (I choose to do deeper linking to give some variety, makes it look more legit than pointing tons of links to your root URL). Then you create some social profiles, add some links there and share them from within. Then you create a few profiles on tumblr.com, reddit, etc and share your sister blog links there, as well as a few from within your main site.

It is a long process indeed, but, it makes the linking look less spammy. From a search engines perspective, if a site has a ton of links to the main url, it is not nearly as natural as having internal links to other relevant pages. the only way to build authority, in my opinion, is to make it look as natural as possible. It can be a real pain, though, especially when you have to continually manage all of those smaller blog sites and articles.

Social media can help improve your rankings. If your content gets shared by many people it will rank better.

This post on quicksprout explains well.