you could also detect when someone on your contact list "joins" a kind20 app by noticing if they post a kind 20. then the app could suggest: "hey, looks like one of your contacts is posting photos, you want to follow them on your kind20 follow list?" 

 Yeah, that's what nostr:nprofile1qqs8y6s7ycwvv36xwn5zsh3e2xemkyumaxnh85dv7jwus6xmscdpcygprpmhxue69uhkv6tvw3jhytnwdaehgu3wwa5kuef0ekucf3 is trying to do on Zapstore. So, you will know which apps to install based on what your friends are using.  

 Family members sharing personal pictures on Olas, for instance.  

 It's an idea, tbc. Many users don't like being tracked like that