At university, I hold office hours for a mid-level CS course, and it is incredibly rewarding to do so in person, as you can see the flicker of understanding on their face when they get it. That's going to be very hard to capture online.

Many people told me, that teaching and mentoring helped them getting better themself.

>But that will challenging to capture online.

Unless there's some barrier to entry it would be difficult, and it'd probably just end up being a spam or meme fest like LinkedIn or Facebook after a while.