Nooooo don't block it!

What will I do during lunch?
As long as the site is monitored & the employees use the site moderately & responsibly, I don't think it should be banned. Plus, you can always update about your site or promote a new post on FB, thereby getting traffic on your site, if you want that.
ORIGINAL: powerpuff
Other companies consider Facebook an essential part of marketing-- to network with others.
I agree with this. And FB is also an essential part of keeping the employees happy. I'm sure many people's happiness & energy quotient will go down when they can't surf the most popular social networking site.
Maybe ban it throughout the day except for a couple of hours in the afternoon & after, say, 5 PM?
One can see this as professional reference rather than personal entertainment, eh.
P.S: Did your office block FB?