Whoa! Happy 5th birthday Twitter - check out what they're doing to celebrate http://discover.twitter.com/
The link directs you to a really cute video with 'famous' people talking about why they use twitter, who they follow, what they're interested in, and so on.
It's a real range of people too, from @SnoopDogg to @RichardBranson and @sfzoo (San Francisco Zoo).
However, one of the 'recommended' people I'm now following with great interest is Paolo Nespoli - @astro_paolo. His Twitter bio describes him as "European Space Agency Astronaut, Flight Engineer of ISS Expedition 26/27 (Dec 2010 - May 2011)". On the video, he comes across as a really nice guy, too!
And his Twitter stream is stuffed full of amazing pictures, such as the recent 'Super Moon' -
Definitely worth a look!