Reddit didn't start with the cloud, though. Very early, they were on dedicated and found it inflexible to their needs, and scaling a site with the eyeballs that Reddit has would have been very difficult with their first architecture.

The "exit plan" is, really, not marrying your entire architecture to one provider. Spreading the love gives you a bargaining chip and flexibility to see which provider will perform better for you in the long run, and allows you to see the strengths and weaknesses of each. Internet latency is pretty bad, though, so sharding an app across multiple providers can be a bit of a challenge.

We're not that stuck. We can be out of Amazon in a month if necessary. We very specifically don't use any of their "lock-in" services to make easier on our open source users, which has the side effect of not locking us in either.