Those very large mysql deployments typically use it as a nosql system, with a sharded database spread over dozens or hundreds of instances, and referential integrity maintained by the business layer, not by the database.
For a good example of a high volume site using a proper rdbms approach I would look at stackoverflow. It can (and has) run on a single ms sql server instance.
Even if that's so, still suggests rdbms are a good choice.
I do know for Wikipedia, english wikipedia is mostly a single master mysql db + slaves, with most of the sharding being on the site language level (article text contents stored elsewhere)
For a good example of a high volume site using a proper rdbms approach I would look at stackoverflow. It can (and has) run on a single ms sql server instance.