thanks for useful information nextelmichaelg. I want to also add something:
lets say you click the option "public website (allow anyone to view this photo stream on the web)" for your shared stream, your photos are also decerased to 2048x1536. I made my photo stream public, it gave me a link, i opened it and i tried to download those pictures from web browser. the result is 2048x1536.
If you want to share your pictures without loosing any quality, you can use photostream but just import those pictures from your photostream when you are on a Mac (i am using imac with iphoto). You will get full quality (i have tested). Do not save them to your camera roll when you are using iphone or ipad or ipod touch or when you are using browser. Just use photostream on those devices to show your firends. So, save your pictures with full quality on your iphoto library, on your Mac, it should be your main place to save pictures.
i have also tested messages app. It also sends pictures with full quality. from iphone to another iphone, or from iphone to imac. it does not matter. it is full quality.
You can also use dropbox or send them via email with actual size.
edit: i took a picrure with iphone and i sent them both to my photo stream and shared stream (to my friends). I import both of them to my iphoto library on my imac (no to loose any quality). the result is: my photo stream has the full quality but shared stream doesnt!!!