Looks like Twitter is getting the attention of Google with the launch of Google Buzz within the GMail interface. A good friend of mine Bruce asked the question on what was different with Google Buzz. Unfortunately there is precious little difference.
Brand perception is probably the main difference if you ignore the small technical differences. When you think of email or search you go to Google. When you think social, its Facebook, Linkedin. When you think noise or realtime, its Twitter. Even Bill Gates uses Twitter. Microsoft also owns part of Facebook, and eventually will take a bite of Twitter.
Its unlikely that Google Buzz will replace Twitter, but I am sure Google will push on to keep Twitterers on the Google platform.
What is interesting is how social networks like Facebook is ushering the slow death of email. In Facebook, sending P2P messages makes email irrelevant. So Facebook is taking a shot at making GMail (among other public mail systems) redundant.
Similarly, Twitter is taking a shot at making SMS redundant. However, it will take 20-30 years to usher in another generation of technology that users are willing to adopt.