* Create a simple pub sub backend message system
* Limit it to 140 characters
* Allow everyone uninhibited API access to it
That's a great public service they've done, but for me, it's not a great business.They've definitely done a great job at appealing to a certain demographic. However, twitter isn't about engagement. It's not about communication. It's about broadcasting 1->many. It's about producers and consumers. That's IMHO what limits its usefulness.
As a business decision, giving away access to 3rd party developers seems like favoring short term growth well above profit and the future. Most people don't use twitter.com which if I was an investor would be worrying.
Facebook OTOH is far more engaging and compelling in its usefulness. You can quickly paste URLs, images, vids, etc. It's more about communication.
I'm not sure being popular should be the aim of a startup. You can be wildly popular for a year or two, then go out of fashion. The people using Twitter aren't particularly known for their loyalty either.