We're allowed to use Twitter here but Facebook is blocked. I'm not quite sure why that is but people don't want to point out the inconsistency in case we lose Twitter too.
Facebook is the only social network banned here but that is simply due to time spent doing persaonl activities on that particular site. Before the ban we were losing 2 - 3 hours per day from a team of just 12 people, so it had to be blocked. No other site has this captivating effect.
However, I think a certain amount of professional/personal blending is desirable, for example it is nice to notice a person you have done business with has other interests.