Why Social Networks are Replacing Email
As to why social networking sites have led to declining use of other communication tools, Pam Horan, president of the OPA, speculates that it's because people can conduct the same activities on the social networks as they did before via email, IM, and other communication properties, but now they can do so more efficiently.
While we would argue that in the business world, emailing is still an essential, "can't live without it" tool, it's not so far-fetched to say that Facebook and the like have changed mainstream users' online behavior. Want to share a funny video? Post it to your profile. Have new pictures from your vacation? Upload them to an online album. These are precisely the sorts of online activities that only a few years ago took place primarily via email messages. Social networking has undoubtedly changed that.