[eitland]

app.net failed because they copied the most useless social network provider of them all.

Note that I specify provider above. Twitter is somewhat useful, but that is despite of, not because of the interaction design of it.

Edit: IIRC app.net was a bit overpriced and had some weird limitations too, although the latter might just be me remembering that they copied all the design mistakes of Twitter for no good reason.

Edit2: If anyone is in doubt at all: Twitter is popular because of the network effect. Everything else has been solved better by one or more competitors.