Is it obligatory to condemn every evil seen on social media platforms like Facebook, and is one sinful for not doing so? And how can enjoining good and forbidding evil be applied in this context?

One must denounce evil to the best of their ability. Whoever sees words or actions on Facebook that contravene Islamic law must denounce them, warn the disobedient, and try to convince them and instill fear in them through verses of warning until they abandon what is forbidden. The evidence for this is the Almighty's saying: "Invite to the way of your Lord with wisdom and good counsel, and argue with them in the best way," and the Hadith: "Whoever among you sees an evil, let him change it with his hand; if he is not able, then with his tongue; if he is not able, then with his heart—and that is the weakest of faith."