What's the difference between in/on a social network? For example, "If two individuals are friends in/on a social network, they ...".
if you speak about an online social network, you have to use "on". For example, on twitter, on Facebook, ...
I would tend to use "in a social network" if I'm talking about a social network in the real world, such as one that formed around people working together, or going to church together, or in some kind of club.
I would tend to use "on a social network" only to refer to an online social network, such as on Facebook or LinkedIn.
That is probably why Ngrams indicate that "in a social network" is much more common.