MariaDB called and said, "I'm still here" ;)

Is it really used/ popular?

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).

At some point I believe Google had the largest MySQL installation in the world and they were using MariaDB.

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.

It's so popular I've never seen anyone actually use the original.

Yes, when most people say MySQL they actually mean MariaDB.

very yes

Yes

I mean, its powering Wikipedia right now and that's the seventh most popular website in the world.

LibreOffice? MariaDB? X.Org?

FreeBSD? NetBSD? OpenBSD? Half of all Linux distros?

Seriously, the number of succesful forks is huge.

FRRouting forked from Quagga. Quagga is dead now, FRRouting is on overdrive.

Like which one? Like the other commenters under you, I can only think of forks that lasted long enough

Probably some survivor bias

Yeah, survivor bias. Most forks die, but there are plenty of successful ones.