It's certainly true that ad-supported platforms can't ignore users wholesale. And a lot get the balance pretty well, like the NYT. If users get upset enough, they will leave.
And in fact, some already have left both Facebook and Twitter. But a lot of others will just create Facebook groups asking Facebook to change their policies. When Facebook does not, they will grin and bear it. After all, its free. Also importantly (and unlike a newspaper), people have network effects tying them to social networks. These make them reluctant to walk.
I think it's silly to say, "It wants money from those users, and all things being equal, those users want to keep their money. So their goals are actually diametrically opposed."
When I walk into a local grocery store, I want to buy food at a reasonable price. They want to sell food to me at a reasonable price. Sure, I want lower prices, and they want higher ones. That's why, like you said, competition is vital.
But they also want to please the person paying them money. That's why they try to help me find things, and bag my groceries for free (some offer to walk them to my car). We both want me to have a happy experience so I become a regular. So we're not "diametrically opposed."
Any ad-supported service also has to try to please the people paying them money (advertisers). That doesn't mean they can or want to stab their users in the back at the first opportunity. But the tension is always there.
With app.net, there are only two masters, users and developers. They have to choose prices that seems reasonable to the user and developer base, as well as their company. After that, they strive to please the two paying groups (whose interests are mostly aligned).
There is no tension between pleasing customers and beneficiaries, since they are the same.
This comparison is a little tricky, since app.net aims to mainly be a neutral provider, almost like a cable company where any conceivable channel can be chosen; in real life, the distributor (Comcast) is a gate-keeper and pays the channel channel (Discovery) a huge amount of money for *content*. With app.net, developers pay a relatively small fee to use the *infrastructure*.
Another key difference is that cable channels still show ads (so it's not a move), while app.net has committed to none on the core service. Some programs using the infrastructure might, but they won't have the same power as FB, Twitter, or if it's successful, App.net.