Instead of just letting people 'like' or 'favorite' users' photos, 500px assigns a score to each photo, from 0-100. They have an algorithm that factors in all the activity on a user's photo to assign that score. As the score increases the photo moves to different sections of the site (Fresh, Upcoming, Popular).

My relative posts a photo almost everyday and is always trying to figure out how to get a higher score; different photo subjects, posting at different times of the day, etc.