Our stack was similar than pinterest, django but postgres and redis. Team of 2.
Indeed, keep it simple. Cache things. Use queue, etc.
You can scale vertically a lot. Db perfs and disk space were our points of focus.
I assume since they're a viral / addictive platform the numbers will be higher. Plus they're not a blog or so, but rather a personalized social media platform, which makes stuff more complicated (you can't just cache their news feeds, they're all personalized).
It's easy to say meh, 11m is nothing, there are other platforms with more users (esp. since you don't bring up examples how you managed otherwise). But I think it's a big technical feat to do this with 6 people.
Basic servers are capable of handling hundred of the housands of those if you optimize your db and use a low level language.
We used python and an orm and we handled 1000 r/s on peak hours daily.
And we are talking user accounts, comments, votes, upload, transcoding, video streaming...
Todays computers are mind blowingly fast.
Afaik a transcoding session holds the entire CPU core, for example malformed gif avatar typically can lie down a single-server web-forum for a few seconds.