On using these type of sites as a backup.. and to add to my previous reply, I was really happy that I saved a file on dA. On Flickr, apparently you can't download the original pic without upgrading to a pro account. But on dA i was able to recover the photo, after accidentally wiping it off my hard drive before backing it up on an external drive.
I use flickr and skydrive for photos I only want certain people to see thanks to the "guest pass" or private links. I tried picasa but the fact that it's sending private links that don't work after taking several hours of uploading photos, kinda ticked me off.
dA for image backup(I use dA's automatic obnoxious watermark feature so i can have my photos protected but still able to download the original file without such a copyright) and for photo critique, tutorials, group chats. heh heh
For me, the site/service being free was my number one priority.
Each site has its features and while paying for a premium/pro account may save time with all the features available on a single account, if you rather not pay, like me, make accounts on several of these sites and use them when you need to.