In general, it's not uncommon for startups of the Goodreads vintage to outgrow a simple Rails app and use a more service oriented architecture. We've actually had services for about 5 years now, but we haven't been very vocal about it.

As I mentioned in another comment I, personally, wouldn't advocate for new teams or startups to use a service oriented architecture right out the gate. It's too easy to end up with a distributed monolith (circular dependencies between services, services uptimes that are tightly coupled). Engineers also tend to underestimate the build tooling, observability excellence and discipline you need to make it seamless.

I'm not really surprised. Goodreads doesn't have nearly the same level of "real-time" interaction that other social networks do. People can only read (and therefore review) so many books, so it's not like there are millions of users constantly posting new content/comments/etc.