In contrast, Facebook has over 10x the people, and 10x the user base, but does major UI shakeups regularly, tries out new features often, and isn't afraid to tweak or remove them if they don't work out. They also are doing interesting things technically (e.g, Cassandra, Scribe) and open sourcing them, some of which Twitter deploys themselves (e.g., well, Cassandra, Scribe).
Facebook definitely seems to fit the startup ideal much more than Twitter, yet Twitter is much smaller. What gives?