Even with a four-month head-start, President Obama's tweet trails Lang by more than 15,000 retweets.

So is it the most retweeted tweet of all time? It has better numbers than any of the tweets on Favstar's "most retweeted" list, but the numbers here aren't perfect. However, according to this list by TwitSprout, it trails a couple examples. A Floyd Mayweather tweet from January, aided by Justin Bieber, has over 90,000 retweets.