I would probably tell him about it simply to avoid a nasty situation later. If he happened upon your FaceBook feed and saw guys hitting on you and people trying to fix you up, and your response is simply to decline instead of say "no, I'm seeing someone," he might think you're not fully committed to him, or that you are somehow trying to keep your options open.
Perhaps I'm the paranoid one, but if I were dating someone on social media and it was serious, and she hadn't mentioned me ever on social media I might wonder. A two minute conversation in language like you used above would alleviate all the wondering.
My $0.02