In the old days companies would have different teams for their social and SEO work. SEO was just about getting found by users and the social team (if there even was a social team) would try and keep those people around.
Now, inbound links from notable social media accounts (likes, shares, +1s etc) can “vote” for quality websites. This is great for anyone trying to find quality articles because it means that the results you get are probably better than ones you may have got in the past.
Websites that Google algorithm ranks purely based on keyword density may not read as well, or be quite as interesting as ones that have been shared by thousands of people. Which would you rather read? An article that has the word you want in it mentioned lots of times, or an article that everyone else who has already read it has deemed it worthy enough to share?
Search engines have also started to measure one’s credibility via their social media accounts similarly to how they measure the credibility of a website. Google+ profiles are appearing next to articles showing how many circles people are in, which suggests how popular / respected this person is.
It is likely that people will end up leaning towards posts by authors who have their Google+ accounts linked to their articles, it just seems more trustworthy. If you can stand behind you work enough to link it to your social networking profile, as well as it being popular then it’s probably a good read.
It isn’t only Google+ links that are getting measured, links that are shared on Twitter and many people see it and a good percentage retweet it; it is likely to show up higher in the results compared to one that has been hardly retweeted.
That being said, does that mean pages that have never been tweeted will be penalised? Probably not, since there are many more factors to the search algorithms than just if it gets retweeted but it seems, getting a lot of retweets is definitely a help. It also means people are actually reading your stuff and like it enough to share it.
Links Tweeted by popular / influential Twitter superstars can be worth something, having communication between those profiles and your own can also make you more authoritative for that subject. This doesn’t mean you should go harassing random celebrities though because they won’t really help you much. It needs to be about your main topics.