Mashable has reported on stats from link shortening service bit.ly that reveal best and worst times to post on Twitter and Facebook to get the most interaction from your networks:
The company revealed that posting links to Twitter between the hours of 1:00 p.m. and 3:00 p.m. ET (or 10:00 a.m. to 1:00 p.m. PT) will give you the highest click rank, especially on days earlier in the week. Meanwhile, sending a tweet with a link after 8:00 p.m. should be avoided — as should posting links after 3:00 p.m. on Fridays.
Facebook's optimal posting times vary - publish your links between 1:00 and 4:00 p.m.; Wednesday at 3:00 p.m. is the peak best time for posting. In general, you should avoid posting important links on weekends.
For more detailed information, read the entire article.