Tent handles photos as "attachments" and doesn't push them out automatically. What it pushes out is a photo post containing the URL from which the actual photo can be retrieved. So your server's bandwidth only gets consumed when someone actually views the photo. It's about the same from a bandwidth perspective as hosting a WordPress photo blog and providing an RSS feed.
Now when you get into the millions of followers, like Lady GaGa has, your complaint is a valid one for textual posts (which are pushed in full). But at that scale, privacy clearly isn't an issue, right? I don't see much use for saying "these 3 million people can see my post, but hide it from everyone else". And if the posts are public, you can use a model like PubSubHubbub's to ease load on the origin server. Tent doesn't have that kind of feature yet, but I imagine they'll cross that bridge when they come to it.
I also disagree with this reasoning:
> There's no way an individual or group is going to be able to host their own servers. The whole system just dissolves down to a single large provider serving the vast majority of people, and we're back to Facebook again.
The first sentence is, in most cases, true, just like most of us don't host our own email servers. There's a substantial economy of scale with email. But can you name me one example of a "single large provider" that monopolizes hosting an open protocol? I can't think of one. With email, we have three large providers (Gmail, Yahoo! and Hotmail together serve ~80% of US users), and we also have a number of small providers. That means you have a choice.