Advice for photo sharing site.
I have tried the first 3 and for non-commercial work I do like Flickr a lot, especially if you use it with Lightroom cause you can use the built in plug in for direct upload/management.
Its very easy to share with friends/relatives and many photo publishing websites such as Photobox will allow you to link directly to your flickr sets when selecting images to print/create products. I am happy enough with their rendering and slideshow features as is, but they are in the process to switching to lightbox so slideshows should improve.
I started looking elsewhere once I put together my professional site and needed to display my portfolios as well as have capability for password protected client galleries, which Flickr just didn't have. Embedding flickr slideshows proved very annoying and not really workable. So that is when I first tried smugmug and then zenfolio.
THese two are very similiar, but I would definitely pick zenfolio over smugmug. I found it more straightforward and simpler to use. The Lightroom plugin for Zenfolio also worked better for me than smug mug and the photos rendered a bit better in zenfolio. Its very simple to create password protected sets that I can forward to clients and also to link your client galleries to your existing website.
Ultimately, though for my business site I ended up using a wordpress photography theme called Photocrati that allowed for superior rendering and more flexibility in terms of gallery structure and display. Here is my site if at all interested and from my blog you can link to my flickr as well www.dumplingsphotography.com
hope that helps!