Facebook seems to be getting more and more popular as "regular" people start to embrace and use it. I know in my group of family and friends I am seeing adoption of Facebook by my church, soccer parents, family members, and more at a rate I haven't seen before. There are several mobile clients out there for various platforms and for Windows Mobile I have been using FriendMobilizer on my Treo Pro. Microsoft just release their official Facebook application that can be downloaded now for free from www.windowsmobile.com/facebook or directly from your mobile browser at mobile.microsoft.com/windowsmobile/en-us/downloads/facebook.mspx and it works on both touch and non-touch screen devices.
This new Facebook application, version 126.96.36.199, from Microsoft offers some advanced features, such as the ability to upload video content from your phone (can any other mobile client do this?) and access to status updates, friend requests, photo tags, wall posts, profile picture updating, and more. The application will be loaded with Windows Mobile 6.5 devices when they are released later this year, but this download works with WM 6.0 and 6.1 devices as well.
I have spent a few days using this application on my Palm Treo Pro and find it to be quite good and stable on my device. FriendMobilizer had a unique UI and gave me some basic info, but I prefer to have the familiar Facebook look and feel to an application and find this new Microsoft client to be better than the BlackBerry client, MUCH better than any G1 Android client, better than S60 (there isn't even a client for S60 that I have found) and a bit more functional (thanks to video support) than even the iPhone client.