Is it sinful to tell friends or social media about what happens in the bedroom?

Yes, it is a grave sin; spreading the secrets of a couple's intimate relations is strictly forbidden.
The Prophet ﷺ said that among the people of the worst standing with Allah on the Day of Judgement is a man who is intimate with his wife and then spreads her secrets. This prohibition applies to both husband and wife. It includes telling friends and sharing on social media or chat groups. The exception is a private discussion with a doctor, counsellor or the court to find a solution. See Etiquette between husband and wife.