I was just reading this great article in the September 2012 issue of Website Magazine: http://www.websitemagazine.com/content/blogs/posts/pages/facebook-engagement-ideas.aspx
Here are some interesting statistics:
Best Day to Post on Facebook: Tuesday. Worst: Sunday
Best Time of Day: 10pm and 12am (Eastern Time) Worst: 1am and 4am ET
Frequency: Between 20-32 per month (And definitely no more than 35)
Format: Photos – with video content engagement rapidly growing. Simple status updates ranked last.
Although these statistics were measured for Facebook, my guess is that email campaigns have similar rates.