Besides using blog comments and social bookmark sites to get backlinks you may also create fake profiles on social network sites. Why would it increase your page rank and your serps?
* Free Backlink to increase PR
Most social sites give a profile page to their members where one can put a link.
If link aren't clickable it is still worth to put a link, hard links do have a value. Example:
Non Clickable: http://increase-pr.blogspot.com/
The non-clickable link won't be lost: Google bots will pick it up!
* How to maximize the Google juice passed on by a profile
Getting a backlink, that's good, but it's even better if the backlink comes from a page filled with keywords related to your niche.
So when you create a profile, select a relevant nickname: if your site is about PR then, chose the nickname Pagerank if available, or try a variation on the theme. Some sitesuse your nickname to build your profile URL: jackpot! That's the best scenario: if the keyword is in the URL too besides being on the page then the backlink from this page will be really worth for your ranking.
An example of profile created for SEO purpose:
So look for social sites and go create optimized profiles in order to increase your Google page rank and to get better SERPs!