Just a quick note for those who take pictures and use Facebook, MySpace, etc. In an attempt to retrieve some of my sanity, I've rejoined a bit of the social world only to find that if I try to share my photos on these sites they end up looking like utter crap. Now, saving space and bandwidth is a laudable goal of course, but destroying the appearance of something one has put some time into is another thing. So, what do you do?
For MySpace, save your images as PNG. For some reason their backend doesn't allow itself to transcode photos, and so once you upload a PNG, it will keep it as a PNG. Keep your image within their sizing requirements (600px I believe) and it won't resize them either. As a note, if you edit in 16bit, do be sure to switch to 8bit before saving, as you'll be wasting a lot of space and slowing viewers' downloads otherwise.
Facebook is another story. It will transcode every image uploaded into JPEG, and insists on recompressing your image regardless of its original size. Your best bet is to maximize JPEG size / quality on the original save to preserve as much detail as you can when it recompresses. Again, you're stuck at ~600px for your maximum dimension.
See a comparison of FB transcoding and recompression here: http://www.facebook.com/album.php?aid=2025397&id=1484857429&l=ecff6205e4.
Update (11/2/10): FB now allows images up to 720px on the long axis. A huge 'thanks' to Ofir Abe for letting me know! I should probably spend more time on FB, huh? :)