For me it's just personal preference, but I've noticed that at times their pages consume lots of resources, more than they should for a simple refresh and I vaguely remember reading an article about something they were doing, but I don't remember the specifics of the article but it was enough for me to just close that tab unless I was actively using it.

They change their code often, so who knows what they are doing at any moment in time. They make money by selling information about you regardless of your privacy preferences, so the more info they can mine about you, including what else you are doing, what pages you visit, what you buy, etc, etc, the more info they have to sell about you. That is their entire business model. That's why their privacy controls are so convoluted.

For some interesting reading, do a google search on: facebook tracking

-David