Facebook has released Facebook Messenger for windows phone.
Main features:
Get to all messages without having to open Facebook.
stickers.
send photos privately.
group conversations and make plans on the go.
Share your location.
Know when people have seen your messages.
See who’s using Messenger and who’s active on Facebook.
Stay logged in so you never miss a message.
Turn off notifications.
conversation history or contacts.
Download Facebook Messenger