At facebook, they have engineers devoted to pleasing users, and others devoted to pleasing advertisers. It's a difficult line to walk, but it's a legitimate argument that advertisers may win in the end because Facebook is now a public company and must show positive earnings growth and all that.
App.net (I think) is proposing that if that line does not exist, they can focus 100% on building a product that users will love using, rather than something that strikes a balance between pleasing users and advertisers. I could see this being a better product in general.
All your concerns are valid though. I'm not on facebook myself, so this service itself has absolutely no appeal to me--however, I do think there are some good points behind the premise, and I've heard through the grapevine that a lot of people do use facebook pretty frequently.
The reality is there's always going to be pressures from different directions, there's nothing special about app.net in that regard.
I wish them well, but I don't expect to be paying for access any time soon.