This infographic by Bitly explains when it is optimal to share content on Facebook and Twitter. This information is especially crucial for brands that want to optimize traffic to their sites. This chart provides valuable user interaction statistics as to the hours in the day that are optimal.
The best time to tweet is from Monday to Thursday, between the hours of 1:00-3:00pm, and on Friday it is best to tweet before 3:00pm, as people tend to head out for the weekend. According to this chart, it would be wise not to tweet after 8:00pm. In terms of Facebook, the highest click-through rates are mid-week between 1:00pm-4:00pm; and traffic tends to accelerate around 9:00am and dies off by 4:00pm.