The best because it helped me organize an undergraduate class mini-reunion that gathered approximately 300 classmates and family. As my classmates entered the bar, I inevitably remembered 60-70% of them and even details of their lives...because of Facebook. It really was a great tool.
And then there were the catcalling and comments directed at me by non-ND folks during the game. I should have ignored them, but I couldn't. I said some regrettable things. I made stupid arguments. I invariably played the role of "arrogant ND fan" in their eyes. It's my mistake more than Facebook's, but Facebook made my decisions much more public.