Has anyone tested the effectiveness of social media signals from Twitter, Facebook, and Google+ as applied to influence on search engine rankings? For instance, is it better to have 100 +1's, or 100 Twitter mentions, or 100 Facebook likes?
Good question, and I do have some stats from my history of using these services to try to help you. I do use Google Analytics and my Web Hosting Provider Stats for giving out the numbers of how much traffic comes from each network.
I forgot to say, the amount of likes, 1 Pluses, etc does not effect traffic. That is how many people thought that the post was good. People who actually view the post to likes and pluses does not reflect the traffic.
Facebook does give me the most traffic of all three of the social networks. I namely advertise to groups which I am a member of. For me, those are the small technology groups. Likes, comments, and views of the Article does provide who is actually reading the article. With my two groups that I visit, I gain an average of 20 readers.
Facebook Pages is a different story. They make you pay for the views of getting your post out to people who have already liked the page. Then, to get your page out to new people, they make you pay for advertising the page to new people. So, I wouldnt recommend this method for pages.
Groups on facebook do work, pages are a money waste, but if you do have a popular page with 2000 likes or more, the stats I got from that was on average for 5 USD in one day only for boosting a post: 30 Likes, 8 Comments, and 10,000 views of that post.
My total traffic from Facebook is 40%.
Twitter is a little harder to advertise on. Information or your post is most likely not gonna be saw by the majority of your followers. Information just flows so fast on their screen thanks to how many people you are following, unless you are on a list which filters out 10,000 followers and has you and a smaller amount of people on. For my stats, I achieved only about 2% of my total traffic coming from Twitter.
If you "boost/pay/etc" your post under a hashtag, then you may get more traffic, but I have never did this and pricing isn't really the best from what I looked into.
When it comes to using it, it is how many people have you in a circle and how many people you have in one also. My traffic from Google Plus is at unknown. I don't think I am reaching anyone, nor does half of my people really care? Not sure here sadly.
I hope this helps you out a lot through what I have experienced. Traffic from Facebook does help a good bit, surprisingly. If I had money to pay for the advertising on Facebook and Twitter all of the time, then we could see a lot more results from it.
A better question to ask is what is the goal of your site. Then ask yourself where does your target audience spend most of their time online and try to get traffic from there.
It is questionable if Google would take the number of likes into account when determining the rank of your website as you can always buy likes.
I would not overstate the power of social media likes, as removing like buttons may actually increase your conversion rate.
I would however use Google authorship as that really has an impact if the searcher has a connection to the author - I did a search for a book name and a friends review came 5th. Did the same search in incognito and the review came 21st.
Do it smart, don't just mindlessly amass likes.