The Bitly blog calculates how long shared links capture people's attention, using the half life of 1,000 popular bitly links:
The mean half life of a link on twitter is 2.8 hours, on facebook it’s 3.2 hours and via ‘direct’ sources (like email or IM clients) it’s 3.4 hours. So you can expect, on average, an extra 24 minutes of attention if you post on facebook than if you post on twitter. … The surprise in the graph above is links that originate from youtube: these links have a half life of 7.4 hours!
The Dish 