I do not run websites and blogs that gets lots of traffic, but it does help promoting your blog. So yes do sign up with all these social media sites, but be prepared to do some work.
Though wordpress.com and .org and other blog services have plugins that will make that easier for you. So you only write your post ones and they pass it on to each others automatically.
The work, is to answer people comments on your post, if they don’t get replied to they will not come back. Keep your little community running and happy and they will come back to read your site. Oh and make sure that there isn’t any broken links missing images etc.