Android: One of the biggest complaints about the Facebook app is that it can take up too many resources, particularly on older phones. This is probably why Facebook released Facebook Lite, a super-lightweight version of its app directed at developing markets.
Currently, the app is only available officially in a few countries -- Bangladesh, Nepal, Nigeria, South Africa, Sudan, Sri Lanka, Vietnam and Zimbabwe -- but there are so many ways to download the APK for yourself, it's unlikely to stay within those countries for long.
The app itself is incredibly spartan, with very little excess. The APK is about 252KB, as opposed to the 27MB that the main app takes up, so if you're on an older device, a slower network, or just don't want all the extra junk, Facebook Lite is probably exactly what you were looking for. (But if you don't like it, the mobile site is always a good alternative.)