Although fluid and responsive layouts are trendy now because of the big amount of different screen resolutions, fixed layouts still have pros:
- Fixed-width layouts are much easier to use and easier to customize in terms of design.
- Widths are the same for every browser, so there is less hassle with images, forms, video and other content that are fixed-width.
- There is no need for min-width or max-width, which isn’t supported by every browser anyway.
- Even if a website is designed to be compatible with the smallest screen resolution, 800×600, the content will still be wide enough at a
larger resolution to be easily legible.
You can see the whole article comparing fixed vs fluid vs elastic layouts.
Facebook is the second most viewed site in the world, and it serves a hugely variable audiance, many of them on underdeveloped countries, so it needs to have a site that fits all kind of screens and, specially, browsers (Facebook stopped IE6 support in Aug. 2010). Most importantly, the Facebook experience must be the same for everybody, it is their trademark that differentiates them from other social networks. If you also take into account that they already target handhelds with OS-specific apps or a mobile web version —which, by the way, is fluid—, that leaves making desktop version's layout fixed the smartest option.