Twitter was back up by mid-day and it was reported that the attacks had originated either from Russia or the former Soviet republic Georgia.
Twitter confirmed the outage was linked to malicious attackers in a brief status message posted around 11:00 a.m EST.
Facebook experienced problems for part of the day, but it was unknown if they were the result of this attack.
We are defending against a denial-of-service attack, and will update status again shortly.
Update: the site is back up, but we are continuing to defend and recover from this attack.