It scales, to a point. In my opinion however, any type of sharding/clustering is still a total mess in PostgreSQL. Most projects will however probably never need to.

How DARE you, sir, imply that we are not the next "Amazon"!

Kidding aside, I suspect with a little discipline, one could design a PG DB that could be ported to Oracle (Exadata???) should the need, and budget ($$$$$), for that sort of scaling arise. Cheap and safe now, Expensive and safe later (if needed)

Also, I seem to recall that PG scales up on multiprocessor hardware fairly well (vs say MySQL), so one can explore exhausting that limit first before more drastic measures are taken.