Nostr is a protocol. Social media apps, and other apps, are built on top of the nostr protocol similar to the way yahoo and gmail are built on top of the email protocols POP3, IMAP and SMTP. Web pages are built on top of HTTP and HTTPS.
Nostr has developed apps beyond social media to include payment networks, podcasting, long form blogging, linktree/resume style pages. The strength of nostr for social media is you can avoid "jumping" from platform to platform having to rebuild your following. Your key pair is what matters, and you can take the keys with you to every platform without losing your prior content.