Full stops are rude?

I have been hearing some surprising small hints about the teen culture in text messaging.

Did you know that it is rude to finish your message with a full stop?  Apparently this is an indication that the conclusion is a somewhat  pointed remark.

And who would have guessed that a smiley face in the wrong context can be taken as a hint of sarcasm? :)

On asking how we are supposed to know these things, it was not very reassuring to be told "it doesn't matter for you because you are old".  Hmm . . . not all that old!  I don't yet know what I want to do when I grow up!

But apparently I am safe, so if you find me committing either of those offences please forgive my age!  I'll continue with my opinion that it is rude not to punctuate your sentences properly.