“Best” depends on your use case. I don’t even think a SQL database is an option for a chat app since it’s write heavy. FB Messenger uses HBase and Discord uses Cassandra and they’ve done their research at scale, so those could be possible options.
Which self hosting instance would go at the size of Facebook or Discord?
Just because Discord took a method doesn't mean a competing product should too.
I am assuming they want to provide a hosted service just like Discord at some point.