Does It Cost If You Use Facebook Mobile Text Messaging?

ok i have a sorta like prepaid plan with us.cellular its unlimited text tho the 60$ a month plan. Buh it dosent have internet buh i still activated facebook mobile. i can like check messages and update my status throught text. So if i have unlimited texting would it still cost ????? someone plz answer this question cuz i really need 2 know plz and thank you!?