[cookiecaper]

People have external incentive to use Facebook and Google. Facebook: keep up with friends and family. Google: find stuff online.

Twitter's only incentive is its use as a broadcast platform for people who are, in general, otherwise famous. People hop on to interact with and read content from them. If large numbers of this group decided that Twitter represented competition and not an asset, they'd dump it and the attraction would go away.

For years the only place I heard of Twitter was television channels desperately begging people to get online and tweet them. It has finally penetrated to the point where most fairly tech-savvy people have started an account and issued 3-4 tweets, but that took years of constant pushing by mainstream outlets.