Social media platforms often function as repositories of our past selves by confronting us with our digital traces and what these say about who we used to be. The digital traces we leave all over the Internet, such as posts and images, comments, and reposts on social media, help us realize just how much we have changed in time. Yet social media platforms (abbreviated as SMPs from now on) do not merely document our self-transformation in time through their archival features; these are also environments that enable the very transformation of the self, spaces where our current selves slowly change into our future selves. This presentation proposes a normative framework for evaluating social media platforms as environments fostering self-formation for their human users. Social media platforms actively facilitate self-transformation, by shaping users’ present and future identities. Unlike offline social environments, social media platforms offer opportunities for redesign and value embedding, potentially steering users’ self-transformation in more intentional ways. The presentation puts forth a nuanced understanding of agency in self-transformation, proposing the situated concept of formative agency. The taxonomy uses three dimensions to map social media platforms’ suitability for fostering or hindering self-transformation processes by looking at kinds of formative agency fostered for users, dimensions of the self changed, and at the modal spaces opened up for users. This normative taxonomy provides a foundation for evaluating SMPs’ role in shaping users’ identities, acknowledging the complex interplay between digital environments and individual agency.