Often I find people in the chat using emoticons. Most are simple to decipher, but I can't figure out what these two mean.
What do they mean?
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As with most smileys, but especially with these unclear ones, how they are being used by any given person might vary. However, this is my experience with them...
:3 is an animal face, especially a cat. It either means "cute" or "mischievous." As in "I'm too cute to be mad at, right?"
:S means "worried". The mouth is all contorted as if in pain. As in, "this topic makes me upset."
:3, as suggested by Jeremy, is a cutesy animal face.
:S, however in my experience, is a confused smile, as you see in MSN messenger.