I am a big user of Facebook almost solely for the messaging system. Trust me, you do not really want to know why. Point being, seeing me friends with iPhones and even advanced Windows Mobile phones being able to post messages in our threads with ease was frustrating for me and my Centro, which had only rudimentary web support for Facebook. I am pleased to note how well the Pre handles Facebook on the web.
For one, it has built in contact support so that once I synced my account, every one of my friends who had a mobile number listed on Facebook was automatically added to my contact list. That has its pluses and minuses, but overall, it is a really nifty feature.
The big plus for me though is the streamlined way it handles messaging. Basically, just like I was at a true computer. The only difference I can detect is that it displays new messages inside a thread in descending order whereas my computers all display them in ascending order (by time stamp). That may just be some sort of Facebook mobile setting that I have yet to find though.
One other thing about my experience with Facebook that I am not terribly fond of is that when I change tabs, for instance going from Home to Inbox, there is no indication that the browser is working and sometimes the loading can take just enough time that I am left wondering if it recorded my intention or not, which can be irritating because if I lose patience and stab at it again, I can end up in weird places and usually zoomed in, which can make it troubling to figure out where in the app I actually am.
Overall, the web browser does a good job of handling Facebook already and I assume the experience will only get much better when a standalone Facebook app comes out