Flickr seems to have a good social aspect but other than that I detest their web interface and don't understand how or why people use it. It seems designed to make it hard to casually browse photos, inserting a painful full page reload in between every image I click. PicasaWeb is miles easier to use. Ironically, I don't really like the Picasa rich client so I use a different tool and configure it to upload to PicasaWeb.