Review: LG Quantum
Facebook is built into the People Hub. From there, you're limited to interacting with wall posts and status updates.
A separate, dedicated Facebook application (found in the Marketplace) is fashioned after the Hubs; users swipe from left to right to access different segments of content. The main segment offers access to all the different facets of Facebook, including your own profile, friend requests, notes, account settings and so on. After that, the next segment houses the news feed, the second offers photos, the third offers events and other calendar items, the fourth offers notifications. This application looks great, works well, and I like the way it organizes friends' Facebook photos into a montage of images. This is great.
As far as Twitter is concerned, no apps are pre-installed, but a number of Twitter apps are available from the Marketplace. My favorite is the official Twitter mobile application. It works very well, and makes the best use of WP7's strengths.