[nkallen]

Suggestion:

In additions to hiring scalability experts (many will claim to be experts but most aren't), talk to your investors to find outside technical advisors who have worked on and are still working on similar problems.

Find advisors from Google, Facebook, and other places who have dealt with these issues. In my experience, you have as much to learn from people who have made mistakes as from people who have succeeded.

Also, in my judgement, you guys are a little too risk-tolerant for your significance (first major Scala 2.8 upgrade, largest MongoDB deploy...).