Mashable.com has a post about the best and worst times to share on social media networks. What I found most interesting about the article is that different social media platforms have different optimal times for posting (as measured by number of click-throughs). For example, Facebookers are best served by posting between 1-4pm EST, while Tumblr users should wait until after 4pm.
Read the full post at http://mashable.com/2012/05/09/best-time-to-post-on-facebook/ (and the original bit.ly post it was based on at http://blog.bitly.com/post/22663850994/time-is-on-your-side)
(also, dig the image I made for this post. pretty clever, huh?)