Hell, it's not even cost effective to switch to another SQL database like PostgreSQL.
Can you imagine the downtime required to export Facebook from MySQL and to re-import it to another database? The users would go ballistic!
I don't expect any "earth shattering" movement by any of the big users in the near future.
I'm involved in a project that involves moving databases. We write each transaction to both the old and new structure using our data access layer, then export historic data and eventually, once we've verified the new system is working as expected, remove the old structure from the data access layer. This is the main reason data access layers are used.