They may have very little functionality, but do it at scale. e.g. even searching tweets (which doesn't seem to work that great...) is a massive undertaking (not astoundingly little) that they're obviously still working at.
That's a fair point and I don't trivialise the engineering however I am taken aback by the just how little product they produce. In a similar timeframe Facebook had created vast swathes of product and dealt with far greater user numbers than Twitter.