I don't think many Internet marketers would question whether or not social signals are used in search. It's pretty clear that they are. The only question is, to what extent?
Facebook and Bing have a very close relationship. In fact, if you are logged into Facebook, then take a look at Bing's home page. You should see along the top navigation menu a little Facebook icon next to your name. If you click your name or the down arrow, then you'll see a link. Click that link and it takes you to a page explaining Bing's social search feature.
In short, Bing allows you to see what your Facebook friends like, not necessarily what they are searching for. And your Facebook friends can see what you like.
Google does something similar with Google+. If you conduct a search on Google, you'll see a +1 icon next to the search results. If you +1 an item and you are logged into your Google+ account, then that item will appear on your +1s list. The same for the people you connect with through +1. Then you can see each other's +1s - if that person has set their preferences to allow you that privilege.
But Google goes one step further. On the search results page, it will tell you which of your Google+ friends have shared an item on Google+ or on any social network they're a member of. So you can really watch what your friends are sharing.
This is just the tip of the iceberg on social signals. There are plenty more, and I think the search engines will get a lot more sophisticated using those signals for search purposes. It will be interesting to watch.