PostgresQL. It scales very well in my opinion, although I have yet to get to Face Book sized numbers :-)
I second all the 'has real features and is more reliable' comments that precede this one; I add to those my number one favourite. I can develop on Win32 and deploy on Linux and apply backups, schema changes, what have you from one to the other without any pain at all.
I started with MySQL but simply could not achieve the above.
YMMV naturally.