Obama's "four more years" tweet and photo didn't just win over Twitter, it won over Facebook as well. His short and simple tweet became the most retweeted ever in a very short period of time. That tweet is now at close to 650,000 retweets, an all-time record by far.
Now that the photo from the tweet hit Facebook, it seems it was the key to the tweet's popularity. The photo became the most liked on Facebook when it hit 2.1 million likes.
It's closing in on 3.2 million now and has over 380,000 "shares" which can be considered the equivalent of Twitter retweets. It's interesting that on Twitter, the tweet has some 222,000 "favorites" which is the equivalent of the Facebook like.
It's obvious that Facebook has the bigger reach, but Twitter's audience is more prone to get involved and share something rather than just clicking "like."