Building a social media app where users can post their selfies and self expressions

This is not permissible. Through you building a social media app where users will post pictures of animate objects, you will be involved in the sin of photography as well as inviting others to commit this grave sin.

There are severe warnings sounded in the mubaarak Ahaadith for those who get involved in the sin of picture making. Rasulullah (sallallahu alaihi wa sallam) said, “The people to be inflicted with the worst punishment on the Day of Qiyaamah will be the picture makers." (Saheeh Bukhaari #5950)

In another Hadith, Rasulullah (sallallahu alaihi wa sallam) has cursed the one who receives interest, the one who gives interest, the tattooist, the tattooed and the picture maker. (Saheeh Bukhaari #2238)

The Hadith also explains that the angels of mercy do not enter a home in which there is a picture of an animate object. (Saheeh Bukhaari #5960)

And Allah Ta'ala (الله تعالى) knows best.