Facebook is an app you use often, so the increase in responsiveness is well worth it.

In the case of most startups, native apps are unnecessary. I already see that feeling trend my geek friends ("Ah, not another of those sites that want me to download their app. I just want to see the [movie listings, items for sale, opening hours, etc.]").

Worth noting, also, that Facebook (and Twitter) is mostly just a window with a browser (hence why the mobile sites are very similar).