Because selling your personal details is a (relatively) invisible intrusion (unless the provider really screws up, ala Beacon) - there is a significant cost to the user (that cares about privacy) that is not visible to them. App.net is aligned only with the users from that position. There is _no_ incentive for them to resell their user's private information.
With regards to advertising - clearly that has to do with your pain points regarding advertising. We've heard lots of feedback from people who say they don't mind the advertising on Facebook, or twitter. These people also might even watch commercial television.
For those of us who stopped watching commercial television a decade ago (before "cutting the cable" was in vogue), who run ad-block religously, and are offended by "kmart specials" appearing in our twitter stream - we clearly have already reached our pain point, and are looking for something new.
For those people - App.net will be a consistent, long term, ad free, communications infrastructure.
There may only be 10,000 or so people there, and the other 5million (50 million? 500 million?) people may be on twitter. But, unlike Facebook, where it's important that all of your high-school friends, ex-girlfriends, aunts, nieces, classmates, and party-goer-chums are on the same site - I'm quite happy to leave them all behind and follow a small core of interesting technical people without distraction.