This helps somewhat from a competition perspective, but the overhead of downloading and uploading potentially GB or TB of data (messages, photos, videos, etc) i

This helps somewhat from a competition perspective, but the overhead of downloading and uploading potentially GB or TB of data (messages, photos, videos, etc) is not something users are accustomed to and is a significant barrier. While it can make switching costs somewhat lower (does uploading your posts from another platform always make sense in a new context with new people?), it doesn't do anything about the network effect.