Is Postgres ready for web-scale dynamic web apps yet?

Postgres seems to be having quite a surge of adoption & awareness recently. At MySQL Conference this year, tons of hands went up for people using or investigating Postgres, and I know their recent releases have seriously moved the ball forward. It's clearly killing it in the data warehousing space, etc.

My question: Is it ready for web-scale deployments? Let me define what I mean since that's such an amorphous term: Could it be (or, better yet, is it) the main datastore for a highly dynamic web-based application that stores terabytes of data and is in the Top 250 (see: https://www.quantcast.com/top-sites/US/3) US websites or some similar metric. Think Facebook- or Flickr-like MySQL installations, but using Postgres instead. I'm not talking about large-but-most-static sites (maybe news organization), etc - a real dynamic web app.

Is there such a beast? If not, why not? Are there pieces still missing? (I understand replication has come a long way, for example, but is it there yet?)

If it is possible, love some concrete examples. Who's done it?