The account that is sending these messages is a fake account set up to send these messages in hopes that you will respond to them via text message. If you do, you will be sent a link to a pornographic site, according to a poster here who did text the number.
If you get a message with this number:
1) Unfriend the person sending the message.
2) Report the account to Facebook as either fake, spam, or scam.
In order to protect your information, as there is potential for identity theft as well:
1) Go to your privacy settings at the top of the page in the left hand corner.
2) Hide the following:
Your phone number
Your email address
Your date of birth (you can hide only year if you'd prefer)
3) Hide your friend list.
Why am I telling you to hide this information? In the case of phone number, email, and DOB, there is no real reason this needs to be on Facebook. Anybody close enough to call or email you probably already has this information stored in their phone or email contact list. They probably know your birthday as well.
As for your friend list, I believe this is how this number is finding new contacts. Hiding your friends list will prevent the people you know from falling victim.