Yeah, they're wildly popular with just about everyone I know 25 and under, especially on sites like Instagram. There's even been a music video created from them: http://www.youtube.com/watch?v=qmlJveN9IkI
Compare the following mentioned during a code review:
Your code is crap Your code is crap :-)
The first is taken seriously, the second is more of a sarcastic joke. The meaning conveyed is even stronger when the picture is a comical and brightly coloured smiley face rather than an ascii smile.
(I'm sounding like a right grumpy old git here)
Emoji can certainly stand in for text, and I see plenty of comments and text messages that consist primarily of emoji. But just as often they will accompany text to add personality or mood the way ASCII smilies or textual kaomoji always have on bulletin boards.
Now that I think about it, you're completely write. It's been years since I've seen text-speak.