How are incremental updates rolled out to a subset of customers on a website?

How do the guys like Google/Gmail/Facebook do incremental releases? For example, on the implementation side, what do the开发者_JAVA技巧y do to roll out features to a subset of customers? I can think of two possible implementations:

1. Effectively put if statements around code on both the front-end and the back-end.
2. If you have customer affinity for certain application servers, you could deploy to just a subset of application servers.

The first just seems kind of hacky to me, and the second seems hard if you are using an RDBMS for your backend store.

---

Multivariate testing is a commonly adopted approach. http://en.wikipedia.org/wiki/Multivariate_testing