You don't need to send them a selfie nor post on Elkbook, the game will automatically do it for you. Just keep going to different places on campus (I usually get it after about 3-5 spots, but it's very RNG-dependent), until RMP tells you the dare's complete, you don't have to go back to your dorm, have fun, etc. (Because you can hook up with other streakers.)
If you've gone all over the map, maybe you just missed it? Check Reminders on your phone to see if streaking's still listed as Current Dare