The optimum number of times to post to Facebook is three per week

"One of the most common questions I hear about Facebook marketing, is: “How often should I post to my page?” And of course, I wasn’t happy that I didn’t have hard data to backup any answer I might give, so I did a little analysis.

I looked at over 2,600 of the most liked Facebook pages and their posts per day rates. What I found was surprising. As pages posted more than once a day they tended to have fewer likes, especially once they got past a 3 posts per day level."

Source:  Dan Zarella in his blog, 13th January 2011
Note - see Dan's blog for a great chart visualising this.