At some point I believe Google had the largest MySQL installation in the world and they were using MariaDB.
very yes
Yes, it replaced MySQL in Debian, and many other distros. The only shared web hosting I know about uses it on FreeBSD, etc. It seems to be more widely used then MySQL at this point.
Yes, when most people say MySQL they actually mean MariaDB.
Unless you actively want the support contract with Oracle, there is no reason not to use MariaDB instead.
Debian changed it to the default quite a while ago, and it's full support for mysql compatibility means you sometime don't even notice it (eg "mysql" is starting mariadb client).
I mean, its powering Wikipedia right now and that's the seventh most popular website in the world.
It's so popular I've never seen anyone actually use the original.