Services like Facebook and Twitter have a functionality called @mentions, but they only work within their own walled gardens; they definitely don’t interoperate–you can’t @mention someone on Facebook with your Twitter account. Why not?! Why not have a simple standard that will allow one website to @mention another–not only across domain names but across multiple web servers and even content management systems? This is precisely what the Webmention standard allows. I can @mention you from my domain running W

Source: Defining the IndieWeb