Most people don't use social as a backlink tool -- there are other benefits that are far better and more direct and focusing on links makes it hard to achieve those -- but if that's your goal, ok.
Blogs are one of the more link friendly networks [facebook is mostly closed, twitter shortens links and the volume flies by everything in milliseconds and google+ remains up in the air although google certainly ranks its own posts] but just because you get a link doesn't mean the link does much good. Blogs that are well managed link out far less than you'd think and a profile on its own, well that's just a single page among thousands on a site with almost no internal weight. Essentially if you're not participating in the blog what you've created just isn't that important.
Comments can be a much better opportunity to get seen [not just for seo, links are after all intended to be clicked by actual people] but that requires providing actual value. Enough not to be moderated out, enough to be valued. But again the volume is generally high on popular places and thus much like a forum one has to assume search engines have a hard time weighting each found response.