It's rarely about growth per se. Inferring someone's social graph and reidentifying them based on that would allow you to link them to their other accounts/shadow profiles and better target ads for them and their friends.

Simple example, let's say the social platform knows you're into football. It has no information on whether your friends are into it, but if they see that you are mutual contacts they can try show football-related ads to your friends too and potentially get a boost in ad performance.