I have the same problem with my Lumia 620 (smiley received as two squares by older Nokia phone) and it also showed up when sharing a photo on Facebook. I posted the photo with a comment that ended with a smiley, but the smiley never made it to Facebook. Its really not very intuitive - I mean as a user I expect the emoticons to work with Facebook if I'm given the option to use them when posting to Facebook. Not a good user experience IMO, but maybe they are very nice if the person/system at the other end is able to interpret them correctly
EDIT - After looking on Wikipedia I now understand that special Unicode characters are being used to encode these emoticons. If a device does not support a character it will be replaced by a square, so the result is quite understandable. However, I still think it gives a bad user experience! I have no way of knowing if the person I am texting will be able to see the symbols and so it seems silly to encourage people to use them for text messages.