A quick search reveals that Elonis v. United States is the first time the Supreme Court has mentioned the phrase “Facebook.” (I am excluding denials of cert involving Facebook). Here is how the Chief Justice describes the social media hub:

Anthony Douglas Elonis was an active user of the social networking Web site Facebook. Users of that Web site may post items on their Facebook page that are accessible to other users, including Facebook “friends” who are noti- fied when new content is posted. …

Eventually, Elonis changed the user name on his Facebook page from his actual name to a rap-style nom de plume, “Tone Dougie,” to distinguish himself from his “on-line persona.” …

Elonis posted an explanation to another Facebook user that “I’m doing this for me. My writing is therapeutic.” …

In the photograph, Elonis was holding a toy knife against his co-worker’s neck, and in the caption Elonis wrote, “I wish.” Id., at 340. Elonis was not Facebook friends with the co-worker and did not “tag” her, a Facebook feature that would have alerted her to the posting. Id., at 175; Brief for Petitioner 6, 9. But the chief of park security was a Facebook “friend” of Elonis, saw the photograph, and fired him.

Over five years ago, Justice Kennedy was the first to use the phrase “blog” in Citizens United.