I had to look that up. I thought that app.net (despite the name) was a twitter clone except that you could write somewhat bigger messages and had to pay for it.

Searched for it on ddg and here is what the excerpt from Wikipedia says: "App.net was an ad-free online social networking service and microblogging service which enabled its users to write messages of up to 256 characters."

Turns out that they did a lot of stuff later that I would have liked but by then I was probably using WhatsApp.