Those same people are regularly out of the loop in conversations their friends have on Facebook, which at times makes them feel a little isolated.
I've heard of one person who made a huge social faux pas because they couldn't read about someone's breakup on Facebook.
The same are all true of some people I know who live on LiveJournal.
Facebook is a great tool for socialisation, and because of that a lot of people use it. When your social circles use it heavily and you don't, you miss out. Yes, you can do all that socialisation outside of Facebook, but the fact remains that a lot of people don't, and you can't really choose how your friends socialise.
It's not an absolute. You're unlikely to become completely socially ostracised because you're not on Facebook, but it can certainly make some peoples social lives more difficult not being on it.
There /is/ social pressure for some people to use Facebook.