I find that 8-10 in the mornings and 4-5 in the afternoon are usually best times for blog posting, facebooking, etc. I do find Sunday afternoon/evenings to be terrific or dead (one extreme of the other) as well.
I let twitter figure it out on its own by using "buffer". You just add your tweets into the "buffer" and it tweets them when your twitter feed gets the most traffic/response. You can also choose to just post immediately.