Facebook Subscribe on YouTube option

One interesting little link I noticed today on a Facebook post by John was a link to “Subscribe on YouTube”.  The link points directly to the correct subscription link for CaseyNeistat which is a perfect “call to action” if a viewer loves your video and then may way to subscribe from within Facebook.

Subscribe on YouTube Link

How do you enable this feature?

It seems that this feature is enabled automatically when you click the “Share to Facebook” icon on YouTube, there doesn’t seem to be another way to currently do this any other way that I could find.

YouTube Share Button

As you can see from the next image the popup Facebook post preview shows the “Subscribe on YouTube” like. This additional feature link is free and will offer a far higher chance of engagement “subscriptions”.

Share on Facebook

So the question is why is this not done automatically and why only if you use the manual share button from within YouTube.  I also wonder why this subscribe on YouTube feature is not yet available for Google+ or Twitter.