Google is like grad-school. People value working on hard problems, and doing them right. Things are pretty polished, the code is usually solid, and the systems are designed for scale from the very beginning. There are many experts around and review processes set up for systems designs.
Facebook is more like undergrad. Something needs to be done, and people do it. Most of the time they don’t read the literature on the subject, or consult experts about the “right way” to do it, they just sit down, write the code, and make things work. Sometimes the way they do it is naive, and a lot of time it may cause bugs or break as it goes into production. And when that happens, they fix their problems, replace bottlenecks with scalable components, and (in most cases) move on to the next thing.
Google tends to value technology. Things are often done because they are technically hard or impressive. On most projects, the engineers make the calls.
Facebook values products and user experience, and designers tend to have a much larger impact. Zuck spends a lot of time looking at product mocks, and is involved pretty deeply with the site’s look and feel.