Format: 

Username (Real Name). "Full Tweet or Title of Post." Title of Website, Date and Time of Posting, URL. Accessed Day Month Year.

Examples: 

@GreatDismal (William Gibson). "Finally read a bit about Mr. Robot. How on earth did someone not manage to stop them using that title?" Twitter, 20 July 2015, 10:32 p.m, twitter.com/greatdismal/status/623319630115766272?lang=en. Accessed 12 Dec. 2017.

In-text: (Gibson)

Notes: 

Rather than provide a title for a tweet, provide the entire contents of the post. In addition to the date of the tweet, provide the exact time (MLA 29). Treat blog posts the same as you would treat an article on a website (MLA 28). If you are citing a comment or untitled post to a forum, you can provide your own descriptive title, but do not enclose the title in quotes (e.g. Comment on "Test Anxiety") (MLA 29). Your citation should start with the author's username (MLA 24) though you can also include the person's real name in parentheses if you know it and think it is relevant to your citation (MLA 102). Because social media content can frequently change (e.g. posts can be removed or edited), you should provide an access date at the end of your citation (MLA 53). As with other electronic sources, omit the http:// or https:// from the citation (MLA 110).

Source:
MLA - p. 29