Facebook can't really risk changing much to the frontend UI else risk alienating the millions of non-techies out there who use the site because their brothers, sisters, aunts, parents and grandparents are on it rather than because it is hip and trendy.

However there is always backend work that needs to be done: security updates, bug fixes, updating frameworks to work with newer tech, automation / pipelining improvements (since they are working on such a large scale), etc. And that's not even mentioning the business side of things like ads, moderation, etc.