Facebook group chats can be really annoying when you're added to one that you don't really want to be a part of or have no need to be anymore. Instead of just letting the message thread run its course and die off on its own, Facebook for iPhone and iPad gives you the option to just leave the conversation.
Launch the Facebook app from the Home screen of your iPhone or iPad.
Tap on the Messages icon in the top middle navigation and find the group message you want to leave.
Tap on the conversation to open it and enter the thread.
Tap on the info button in the upper right hand corner.
Here you'll see a description of the conversation, who's in it, and other information. Scroll down to the bottom and tap on Leave Conversation.
You'll now be prompted to confirm that you'd like to leave the conversation. Tap Leave one more time.
That's it. You'll no longer receive messages for this conversation. Alternatively, in the messages information section you can turn off notifications instead of leaving if you'd like to still be able to read the thread but just don't want to be notified of new messages.