I don't think they had any patents protecting them, etc.
Agreed. Pownce and FriendFeed were both hailed as Twitter competitors in the early days. In 2008 Twitter was consistently plagued by downtime as they had trouble scaling. However, the service was simple and easy to use and they did a great job getting early digital influencers and celebs onboard evangelizing the platform. When Twitter first got started they also had a suggested user list and recommended lots of famous people and digital influencers to follow. This created a bit of a self-fulfilling prophesy - those on the list would evangelize twitter helping to funnel even more people in and boosting their own popularity, etc.
There was competition in the microblogging space and there still is, Twitter just had the "it" factor. People adopted it instead of other services. Developers adopted it instead of other services. The search features were better.
They just had the right mix of factors to beat everyone else.
It's less about the service and more about who's on the service.
On April 17, 2009, at 10:11 AM, Oprah Winfrey tweeted: "HI TWITTERS . THANK YOU FOR A WARM WELCOME. FEELING REALLY 21st CENTURY ."
And that was it.
Twitter servers started crashing every fifteen minutes. A piece of artwork known as the Fail Whale was a constant fixture preventing the rest of us from carrying on our early adopter conversations.
Then, other celebrities started taking it seriously.
Then, journalists started showing up.
There are a lot of other services that work like Twitter. Some of them are even free and open source. Some, like App.net, have paid membership levels.
They just don't have the same people on it as Twitter, and that makes all the difference.