Some guys came up with a web service called Twitter that would facilitate sending group text messages. They imagined people would use it to help coordinate in-person socializing — for instance, you could send a message to your group of friends saying you were at a certain bar, and they could come by. In principle, that seems like a useful service.
As it turns out, though, people started using that service for completely different purposes, because it turned out to be a flexible and convenient way to communicate brief thoughts, links, etc. Often the platform actively impeded the uses people were finding for it, and the implementation of supplementary features (such as discussion threading) was slow and inconsistent.
Now there’s been an IPO and the people who created this service are multi-millionaires. The people who actually turned Twitter into what it is, however, get nothing — unless you count the increased number of ads.