It was discovered recently that the achievements Chatter (say 2000 words in chat) and Chatty Mouth (10,000 words) weren't working properly. I've applied a correction tonight and re-imported the entire content of the chat logs to ensure that everyone's words were counted properly.
If you talked enough to earn these yet didn't score them already, they'll trigger the next time you join any of the worlds for a few minutes!
UPDATE: Make that three, I forgot about Do You Hear Me Now? (50,000 words)