Well my first question to you would be in regards to how you are exporting from iPhoto. I find it's best to export at full size (don't allow iPhoto to downsize the images). Then upload the full size image to the site.

That said, the site is likely going to scale down your photo and perhaps add a little compression to it.

In my tests with Flickr a few moments ago, I didn't see too much 'pixelation' but I did see a little more compression in the image I uploaded. Probably almost too much compression.

There's really nothing you can do once you've uploaded the photos to these sites, as they have their own procedures for what to do with the images once you have uploaded them.