Paypal.com down
twitter.com
twitter.com
It doesn't, really. People decide who to follow. Think of it more like a flashmob than an organization. Some people just start talking loud, and as long as they're reasonable, people follow. If they're not, nobody pays attention.
Cousin comments indicate that it may be http vs https, which is at least confirmed in my browser: https://paypal.com loads while http://paypal.com doesn't (and with the www included in both)
EDIT: Perhaps this wasn't worthy of an upvote, but I'm having trouble finding inflammatory/offtopic/bad logic to give it a downvote. I'd welcome some discussion, though
http://www.paypal.com redirects over to https. It's only http://paypal.com that's not coming up.