As of 2014, in order to send messages to a mobile from Facebook, a user's mobile carrier must support this feature. After signing in to Facebook, locate Settings at the top right of the page, click Mobile, add the phone number and select the country and mobile carrier.
Send the letter "F" to 32665 to activate this service. To send text messages to a mobile from Facebook, the receiver must have enabled this feature. Users can reply to messages and comment on posts using their phones. Facebook does not charge for the messaging service, but the mobile carrier's rates apply.