Search engine results are very important also for the well-known brands, and life saver for unknown ones. I've seen many users while they were using G Search and typing facebook every times when they wanted to visit the site, instead of bookmarking it, or using the URL field at least. It's terrible to see, but makes me down-to-earth. We tend to think everyone aware of the basics and more, because we (the early adopters) are moves routinely in this field. But why we expect that the best cook knows how these things work?

Mention notification is a long waited feature. It's always good when something is built-in. I don't want to repeat myself, I could write the same reasons as above.
I used 3rd party service for that, and the built-in future will make it more widespread, which will generate more interactions for sure. (Abuses raise questions, though.)
Will we get a tab like Blog, Comments for mentions too?