Earlier quoted context omitted.
But as your web application database grows more complex and larger, how do you intend to make them keep up with the 2k req/sec load?
Why would anything need to go to db? :/ Going to db should always be a last, final resort. People often seem to think a db is the only way to write any webapp, and that everything must depend on the db. That's bad architecture. In the case of Mibbit, db access is only done for writes, lazily, in the background. The bottlenecks for me, are CPU, network, and ram. And yes, you can throw servers at it, if you're frivolou…
As far as relational DBs being the problem, that's more a matter of the free ones not cutting the mustard. Oracle and DB2 can handle jaw dropping load in the hands of an expert.