Definitely a tool or at least, mostly a tool - I continue to learn a lot from the site, largely from the comments but also from the links to articles, etc. I suppose it's important to define tool and entertainment. As I see it: A tool is something that helps you grow, create, learn, etc. Entertainment, on the other hand, is something with no intention of helping you, it's only for gratification purposes.

For instance, I think, mindlessly scrolling through reddit.com is a form of entertainment. But if you are subscribed to only certain particular subs in order, for example, to become a better writer, that would be a tool. This is a shaky example though.