Twitter/FB backlinks good for SERP?

Does anyone know if Twitter backlinks + Facebook backlinks are useful for SEO? They are all no-followed, but my site's Twitter and FB page have PR 5, which is even higher than my main site.

If Twitter/FB links are good, what other social media sites should I be targetting?

Thanks!

Google takes tweets directly from Twitter as raw data instead of crawling the site. It's treated as a whole different type of link, but it does count. They're vague on the specifics of how it works (as always) but there's something in place that counts tweeted links from high authority users more than from average people.