Are "post answer" and "vote" an extension of browsing? Since you have to SELECT / VIEW (?) a question in order to answer or vote. Or can I simply say in my use case text that you must have selected a question as a precondition? Should I also change Browsing to search only?
diagram info : I try to depict a basic uml use case diagram for a system really similar to the stackexchange websites. Just consider a simplified version. I know there are many ways to draw these diagrams but I did my best to declutter and keep only the high level functionalities. Authorised user is someone who is logged in. Let's suppose that you can only browse and search for questions if you are not logged in. Credentials are mostly for checking the reputation.