Zivush - 9:18 am on May 25, 2012 (gmt 0)
Social media like buttons (FB, Twitter, Google +) have a very negative impact on page load time - Page speed is becoming a indirect factor in rankings. It could also be a direct factor.
On the other hand, these buttons increase 'social awareness'. I don't think any SEO knows how # of likes affect the ranking of a particular page.
I have seen some brands removing these button while other (news sites) still using them, and tend to remove them as my sites are evergreen ones.