Tweeting later in the day and later in the week results in more retweets.
The click-through-rate on tweeted links appears to spike in the late morning and then again around 5pm.
Still, tweeting more frequently leads to more followers, but Facebook is a different story: Too many posts on Facebook can more easily alienate people.
More people tend to open marketing emails in the early morning than any other time during the day. Weekends also appear to be effective for emails.