Of the photo sharing sites I've seen, Flickr offers the best overall experience for my purposes. I actually like that they haven't done much to change the format, as it means I don't have to keep "retraining" my parents and other relatives on how to view photos of their grandkids. Uploading through Lightroom is a breeze, and being able to reupload after changing a photo gives me huge flexibility. I can upload a draft for quick sharing, then fix elements later (especially handy for scanning negatives with dust or scratches). If the photo's EXIF info is done correctly, Flickr will display the three main data points that I want to see -- when, who, and where -- without having to dig into pop-ups or other pages. It also makes assigning photos to multiple albums ("sets," in the lingo) terrifically easy. I could go on...
I'm sure Flickr isn't "cool" anymore, but for me it's pretty close to perfect.
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