population $p$, penetration $v$ (consider view count of a YouTube video), friends $f$.

Define: $P(\t{seen video}) = v/p\\ P(\t{your friend}) = f/p$

You are interested if you can use a cultural reference among your friends, consider this value $P(\t{seen video}|\t{your friend})$

Assume independence (obviously wrong, however, we have nothing better in the zeroth approach): $P(\t{seen video}|\t{your friend}) = v/p$