After years of being overlooked, there's recently been a lot of attention paid to Facebook's iOS app and it's over-use of background resources. Facebook has indicated they're aware of – and working on – the problem. Let's all say a word of hopeful thanks for that.
Unfortunately, it's not just your iPhone that's being overworked by Facebook. Facebook runs rampant when its website is loaded in your Mac's web browser, too, using more RAM than just about every other app – and definitely every other web page – you have open.
After only being open – and left in the background – for twenty minutes, Facebook had consumed almost 700MB of RAM, for no good reason. Presumably this is because it's constantly auto-loading new content and not jettisoning old stuff. If so, this problem is fixable. Twitter's web page used to be like this and then they changed their auto-loading scripts to be quite a bit friendlier in the resource-usage department.
Facebook had only been loaded for 20 minutes; those two Google Docs have been open for days,
and allow a significant chunk of their used memory to be compressed.
The solution for this is simple: don't leave Facebook open in a browser tab, even in the background. You'll still get most of your notifications in Notification Center by enabling them in System Preferences > Notifications > Facebook.
Enjoy using your RAM for other things.