[eshvk]

Yes, Google is working on world changing problems. You are also incorrect in thinking that all Facebook is doing is work on social networking. They have so huge an engineering challenge that there are enough interesting problems.

However, the key issue in both companies is that they are both at such a huge scale that they have enough crud work that you might end up working in if you are a junior engineer. Just because some of the greatest problems are being worked on doesn't necessarily mean you will work on them.