If you haven't seen it already, it's coming.
Facebook is testing out Youtube 'subscribe' links on videos posted to your news feed. When you're on Youtube and click the 'share on Facebook' icon, it automatically shares the video to your timeline.
Now when you do that, your friends will see a 'Subscribe on Youtube' link under the video on their news feeds. If you choose subscribe it's free, you just need to have a Youtube account. It's free to set those up too.