Facebook, Google, LinkedIn, and Twitter all use MySQL for at least some of their data management. I know this because they all contribute some of their MySQL customisations to a project called.
“One last thing though. Simply because these big companies use these technologies, doesn’t mean that you should. They have tons of engineers and resources, and very high demands, so they can both afford to use multiple databases (which requires specialised engineers) and need to use it (because of the extreme demand). As a small-scale developer, you probably don’t need to worry about the extreme demands, nor do you have the means to use multiple databases.” Source