Personal communications, including email and text messages and direct messages sent through social media, are usually cited in the text or in a note only; they are rarely included in a bibliography. Most such information can be referred to simply as a conversation, message, or the like; the medium may be mentioned if relevant.
Elements of citation
Originator of the communication -- Medium -- Date
18. Sam Gomez, Facebook message to author, 1 August, 2017.