I used to date girls on Tinder with at least a few degrees of separation (ie, no friends in common), didn’t have Tinder and Facebook connected in any way, and the dates would still show in my friend recommendations list.
It was WhatsApp.
It was WhatsApp.
Tinder and a lot of others apps share information with Facebook about what you do by using their SDK.
What I did do is add those girls’ numbers on WhatsApp - which is a Facebook property - and they suddenly appeared as friend recommendations.
But if there is any definitive proof it’s that I only was recommended the ones that I had messaged on WhatsApp, never those that I had only chatted up on the Tinder app...
But it may be very possible that they do a combination of everything to track you (whatsapp, instagram, facebook, oculus, apps that use the sdk...)