Tumblr - after 4pm Eastern Time Friday
Twitter - between 1 and 3 pm Eastern Time (never post after 3pm Friday)
twitter Suite and Business Social Tools
Facebook posting is best done between 1 and 4pm Eastern Tuesday, Wednesday and Thursday.
According to researchers, "...links have, on average, a 3 hour half-life (the point at which the link has received half of all clicks it will ever receive)."
Mashable: Monday is the worst time to post and tweet