Facebook's new mobile iPhone app is a considerable upgrade over its previous one. I won't do a review of the app, but it offers much more utility than before (including chat, notes, photo uploads, etc). We'll continue to see new features and various enhancements in future versions too (next up is probably video uploads). There's also been some speculation that it could become a mobile apps platform within an app (others are pursuing this approach). While all but a relatively small number of highly successful Facebook apps basically languish online, mobile offers an opportunity to reinvigorate the strategy.
There's also Facebook's payments strategy, which could expand to mobile (it's already got a relationship with Zong using mobile phones to pay for virtual goods online [watch for a potential acquisition of Zong by FB]). Then there's the recently announced expansion of Connect from the iPhone to the broader mobile Internet.
Facebook also just announced a relationship with Nokia integrating Facebook (via "Lifecasting with Ovi") into the N97 and new Nokia N97 Mini phones. Here's a promotional video showing how it works:
The company also said in August that it has 65 million users globally who access the site from mobile devices. That's reportedly more than triple what it was in December, 2008.
Clearly mobile is a very key part of Facebook's strategy now and could make it a dominant globaly player in mobile across a number of fronts, including, potentially advertising. It's probably only a matter of time before Facebook becomes an ad network. Indeed, if I were Facebook I would take a look at buying one of the leading mobile ad networks top accelerate that development.