I work at IMVU as well, know the author personally, and can tell you that yes, he unit tests too. In fact, like all of us, writes all code test-first.
For test strategy in general, you need a whole seminar. There's a good blog on testing over at http://googletesting.blogspot.com/ <- go there!
As for code:test ratio, I'm not sure what metric you want, but using just php files, there are ~1k active test files and ~7k php files total. So if you take into account that 1k of that 7k is the test, and some is test-setup and obsolete files we no longer use, it's maybe about 5:1 code:test. However, a lot of the code is third party OS software packages. In-house code is about 1.5:1 test:code, in my experience. These, by the way, are very rough estimates.
For test strategy in general, you need a whole seminar. There's a good blog on testing over at http://googletesting.blogspot.com/ <- go there!
As for code:test ratio, I'm not sure what metric you want, but using just php files, there are ~1k active test files and ~7k php files total. So if you take into account that 1k of that 7k is the test, and some is test-setup and obsolete files we no longer use, it's maybe about 5:1 code:test. However, a lot of the code is third party OS software packages. In-house code is about 1.5:1 test:code, in my experience. These, by the way, are very rough estimates.