every interaction that additional user makes with tweets viewed by those 100k existing users requires 100k updates pushed to those 100k users’ pages. Every like sends an update of +1 like on the tweet to every one of the 100k users. It becomes significantly more expensive per user engagement.

This is not how a reasonable API works and is probably not how Twitter “pages” work. A “+1” just changes a number in a database from, say, 629644 to 629645. The latter is no more expensive to send over the wire to a client when it requests data.