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