I realize this was a while back but what you can do is activate wireshark at the same time your running dsniff/ettercap/whatever.
Then once you've logged onto myspace/gmail/etc you can hit ctl-f to search the packets in wireshark.....select string and search for "password". You should find the username/password in the raw packet data in wireshark.
Hope that helps, not sure why but ettercap has the same issue and doesnt display passwords for hotmail/myspace and a few others....wireshark gets'em all.
Good friend of the forums
if you are trying to sniff passwords just use ettercap -g it does it all for you