Does the noble Hadith: "It is not lawful for a man to forsake his brother for more than three [days]" include friends online, and is the questioner sinful for a two-year estrangement from her friend?

Friendship on Facebook is merely a follow, and the rulings of connection and abandonment do not apply to it. Therefore, there is no harm in not responding to one of them or deleting them. As for true companionship and gathering, the absence of greeting in it is a forbidden abandonment. Abandonment is leaving off speaking to one's brother while they meet, turning away from each other, and refraining from giving salutations.