It's an interesting alternate perspective that has somehow gained the force of common wisdom without the evidence to support it. On a service like Twitter, the users are both customer and product, as five minutes of objectively clear thought would show.
App.Net make succeed or it may fail, but it won't be because of this issue.
I hate adverts, but I think app.net is unlikely to topple FB or Twitter in the social sector.
Myspace is a warning to Twitter and FB, if you don't look after your users, improve your service and keep them happy then you the advertisers will leave too.
Sure the advertisers are a force that doesn't necessarily align with the users, but this is something the people running the network have to mitigate, because it isn't in their intrests to piss off their users.
That being said, I think advertising is in a bubble, so from that perspective app.net has an advantage.