The most popular service is Twitter, which has come under fire in the past for instability and network issues. Since about early August 2008 it seems to have mostly ironed their issues out.

Twitter accepts messages, or “tweets” via the web page, Text (SMS) message and also through a variety of third-party services and downloads such as Twhirl.

Some of the larger news organizations are using twitter, including the New York Times, CNN, and NPR and several businesses. There’s even a famous story of a disgruntled client of Comcast Cable being contacted after tweeting about her dissatisfaction.

Personally, I think this is a huge opportunity for the travel industry. Imagine posting a special with 1500 or more followers. It’s free, easy – by design – to use, and has huge potential.