Not everyone on Twitter uses their real name. Meanwhile I knew the real names of about half the top 20 most active users on a retro gaming phpbb board in the early 00s and had meet many in person and knew we everyone lived, what other hobbies they had and what they did for work or school.
Real names were absolutely used on Usenet especially in the early days, ditto for mailing lists (and still are for that matter), even though technically they are pseudonymous. In any case pseudonymity doesn't seem like it's relevant for whether something is a social medium or not – many social media are pseudonymous (or even anonymous, like the chans). HN is pseudonymous. Reddit. Tumblr. The various Fediverse services.