From Google:
"Does +1 affect my site’s performance in search?
Content recommended by friends and acquaintances is often more relevant than content from strangers. For example, a movie review from an expert is useful, but a movie review from a friend who shares your tastes can be even better. Because of this, +1’s from friends and contacts can be a useful signal to Google when determining the relevance of your page to a user’s query. This is just one of many signals Google may use to determine a page’s relevance and ranking, and we’re constantly tweaking and improving our algorithm to improve overall search quality. For +1's, as with any new ranking signal, we'll be starting carefully and learning how those signals affect search quality"