Review: HTC Arrive
Facebook is built into the People Hub. From there, you're limited to interacting with wall posts and status updates.
A dedicated Facebook application is also available, and is fashioned after the Hubs; users swipe from left to right to access different segments of content. This application looks great, works well, and I truly love the way it organizes friends' Facebook photos into a montage of images. This is great.
As far as Twitter is concerned, there are now several options. First is the HTC-made Twitter application called Twit. There's also an official Twitter client for WP7. The HTC client is much better than what was available at launch (i.e., it actually works now). The official Twitter client is solid, and covers the basics such as @replies, DMs, and hashtags.