Although there are many social photo sharing sites to choose from, lets look at the 3 major players (flickr, 500px and viewbug) and the one huge downfall I have found.
flickr, 500px and viewbug have been around for many years and have also gone through many overhauls and feature revisions. In their current state, the user interfaces are intuitive and the photo presentation is great. All 3 do a great job at displaying your work to the rest of the world.
The downfall I have found with both flickr and 500px is they seem to be a popularity contest instead of a place where you can post your work and get noticed or even get an honest critique. You will also need to post your work at certain times if you even want a chance of being noticed. Posting earlier in the morning tends to have a greater viewing audience than posting in the afternoon or night.
The other day I posted a photo to 500px at 9:00am and within 24 hours it was scored at 98.6 on their scale. I then deleted the photo and the following day posted it at 1:00pm. The photo then only reached 34.3 on their scale after 24 hours. As an artist, this tells you 2 completely different things. One day your work was awesome and the next day it sucked! I think 500px is a little bipolar for my liking.
I have also found the same with flickr as well but not to the same extent as they do not rate your work. Posting earlier in the morning on flickr gets you more favourites and comments but that about it.
Viewbug is a photo contest website that does a great job of displaying your work as well. The voting system is meant to keep everyone honest meaning the numbers are true. If you have a photo that you think is great simply submit it to a contest. This way you can get an honest feel for your work by people voting for it with a voting system that works rather than one word comments or ranking systems that rely solely on site traffic. I think viewbug has it right and will continue to grow because of it.
I will still continue to use all 3 as they do have slightly different purposes. On flickr and 500px, you can license your work or sell it with a free account where on viewbug you can not without upgrading your account to a paid account.
In the end I think my plan will be to post images to viewbug and the ones that rank the highest in contests will go on 500px and flickr. Lets see where it goes!
Disclaimer: I do not recommend using any online system as a sole critique system of your work, but it will give you a good idea of how your work is perceived. For truly honest critiques and thoughts on your work, I would highly recommend finding a mentor in your genre or joining a photo or camera club. This way you can gain insights on your images from people you trust to give you an honest answer.