Astronauts have been tweeting from space for a few months now. However, previously, tweets had to be emailed to the ground, and input via staff on Earth.
TechCrunch: "Astronauts aboard the ISS received a special software upgrade earlier this week, according to a NASA statement released moments ago: personal access to the Internet and the World Wide Web via the 'ultimate wireless connection'.
This personal Web access, called the Crew Support LAN, takes advantage of existing communication links to and from the station and gives astronauts the ability to browse and use the Web. The system basically provides astronauts with direct private communications."