What Is 'Via SMS' on Facebook?

Notifications
SMS notifications deliver a text message to your mobile device when incoming communication takes place with your account. Once you enable SMS notifications, they are delivered automatically until you manually disable the service. To change your SMS settings, go to the 'Account Settings' menu and click on the 'Notifications' tab. Check the box in the 'SMS' column next to a particular type of communication to enable the alerts for it.
Communication Eligible for SMS
Not all communications on Facebook are eligible for SMS notifications. Basic communication, such as incoming private messages and new wall posts, can generate an SMS alert if you enable the service. Less common communication, including being mentioned in a comment or family relationship confirmations, cannot initiate a SMS notification. Other Facebook communication eligible for SMS notifications includes photo tags, photo comments and comments on a link you posted.
Messages
A copy of the private message you send to a friend may also be sent to her phone via SMS, provided she has enabled mobile texts. After you compose your message, check the box next to an image of a cellphone, located just under the message field. When you hover your mouse over the box, a text bubble says 'Also send as a text message.'
Enable Mobile Texts
Before you are able to get SMS notifications or receive a copy of messages, Facebook must link with your cellphone. Go to the 'Account Settings' menu and select the 'Mobile' tab. Click the 'Register for Facebook Text Messages' link and provide your cellphone number and provider. Send a text to the number provided by Facebook, and Facebook sends you a confirmation code. Once you enter the code on Facebook, your cellphone and Facebook account are linked for SMS services.