It's SO simple
Millennials are basically retarded narcissists right?
Make a platform that gives the illusion of interaction, but is really just 100% bots. The bots pour praise on every post saying everything is wonderful, and that the user is a special snowflake, even if what they post is complete drivel. The bots can occasionally emit a pop-culture reference.
Then have the bots post the occasional sponsored product recommendation, and also increase the tweet size to about 500 characters.
Done and done.