Richard Branson & Bill Gates can be mentioned by anyone on Linkedin, even if they aren't your 1st level connections.
Mentioning is a great feature. And if you use it well, the mentioned ones might even read, or comment on your post.
Here is what Linkedin has to say about this cool feature you have:
Linkedin have some great tips how to best use the mention, and some good to knows:
It is good to know, that you can control the mentioning and also the tagging on Linkedin.
There are even more possibility of controlling the mentions and the tags, controlling the visibility of them.
Would like to know more of Linkedin?
Have more ROI?