Facebook uses MySQL because that's what they started with (it was free). They have expressed many times their issues with MySQL and with switching RDBMSs. gigaom.com/2011/07/07/
MySQL is probably the best option to store large amounts of data. Facebook uses MySQL, and you can only image the amount of data they have on everyone!
The benefits of MySQL:
1, Best for beginners
2, The "goto" database language for PHP
3, Massively open source
The biggest thing to keep in mind the your structure of your database. You can handle an uber amount of information, but just keep in mind that setting data types can, and will, restrict the amount of space for specific columns.