Windows Phone some times having an issue in connecting to the default chat Application. Sometimes chat can’t be connect notification will be shown. If you are experiencing the problem, please do the following steps.
Step 1 : Remove the Facebook account from the email+accounts area in the settings.
Step 2 : Go to the battery saver area in the settings and turn off the battery saving mode.
Step 3 : Restart your phone
Step 4 : Add your facebook account again
Step 5 : Go to Messages and then Message settings then turn on the facebook chat.
Wait for sometime, Facebook chat will be available only after a few minutes.
Now the problem may have solved.