Unity's great and all, and works pretty well on the iPhone, but for Facebook there are two near-fatal flaws.
1) Technical flaw: it always drops on top of other elements on the web page it's on. If you want to do a facebook "post to wall" fake popup window thing, you'll probably have to work around it by reducing the size of your unity window to 1x1. There might be ways around it by doing your facebook requests with your own UI, but I'm not entirely sure if that's possible/allowed.
2) Non-technical flaw: Users don't like installing plugins. You're not going to hit super high numbers by using a Unity game. This might change in the future, especially as more big-name people start using Unity for their web games (WB and EA in particular use it pretty frequently).
That being said, I'd trust Unity over the Flash cross compiler for iPhone dev any day of the week. It also has Android support. It doesn't have Linux support, and it most likely will never have WP7 support.
For reference, our game 2D game, Splode, was shipped to the following platforms: iPhone/iPad (universal app), Mac app store, and android apps store. Cross platform dev was pretty easy, all things considered.