jquagga: The photo contest link above should go to connect.dpreview.com/contest I think instead of back to this page.
One thing I find interesting about the contest is that rule condition (c): namely - (c) his/her Photo has not previously been distributed through any media channel; pretty much means I wouldn't be able to enter any of my already connected photos. Since wouldn't pretty much any social photo sharing service be a media channel?
Not that I'm not going to enter and I realize this is probably for legalese reasons. Just sayin!
Another is issue is this: (i) he/she has received the written consent of any person (or parent or legal guardian if such person is a minor) who appears in any Photo.
How on earth when doing candid photography or street photography are you expected to get written consent?
For that reason I can't enter.
IMHO I suggest that which site(s) you use depends on your needs. If you just wish to share 'snaps' with friends then FB is probably the way to go (if you are on FB). if you wish to show your photos off in the best possible way to the widest audience then Flickr is probably the best. if you wish to sell your images then I suggest you look at RedBubble, Deviant and Zazzle. If you are on G+ then it seems that its better than FB at showing off your images but then your audience may not be as great as FB.
Which one is best for you depends on your needs and existing networks, as well as issues of whether you really want to share hi-res images with anybody and everybody.