One of my first forays into posting online, was submitting images for the Mirror Project https://mirrorproject.com/. The Mirror Project ran from 1999 to 2006, and accepted contributions from photographers, showing at least a bit of themselves in a reflective surface. At first I felt constrained to mirrors, the more I looked around the more reflective surfaces I found. Creating content within the rules of project helped me to see the world in a different way, drove me to see things I had previously overlooked.
Sadly the project stopped in 2006, when as the website says, "someone dropped the server." It has been revived in archive form only, no longer accepting submissions.
Still when I saw my orange shoes reflected in the stainless steel surrounding a fountain at the Hirshorn I couldn't resist.
What was your first online platform?