WhatsApp, similarly, had 30+ employees when they got acquired [0]. They built the fastest IM on the market with 450M+ users sending 1B+ messages everyday, and at one point surpassed Facebook in terms of number of images uploaded.

The engs they had were world-class, so really, I think, saying microservices (or latest-fad) get in the way etc is disingenuous since you also require world-class talent to begin with (if you're going to keep the team-size small and yet be able to manage crazy scale) and a competitor willing to pay through the nose for the acquisition.

[0] https://www.sequoiacap.com/article/four-numbers-that-explain...