I know I signed up for Twitter shortly after it emerged at SXSW in March of 2007. I did not start using it until months later. So far the oldest Tweet I can find is from November 8, 2007 courtesy of TweetScan:
My day would go better if I couldn’t count the seconds between clicking and having my co mputer respond. 6:24 AM Nov 8th, 2007 from web
Twitter only let’s me go back in my profile to 5:28 PM Apr 10th, 2008. A day ago I was able to browse back to 11:57 AM Apr 10th, 2008. I suspect that Twitter limits you to going back 160 pages in your profile (ie. history) so those who Tweet less, can see further back in time. I suspect this is why some people on Twitterholic who Twitter more infrequently appear to have been on the system longer than those who Twitter frequently.
Tommy following lead from other manager and making a second attempt. He bumped into an old friend. 11:45 AM Jul 23rd, 2007 from txt
I wonder if there’s a future in Twitter Archeology or Twitter Detective work. I personally believe everyone’s first Tweet is something like "Trying this out. Not sure I’ll use it."
Update: Twitter Tip- Favorite your first Tweet to make it easier to find in the future.
Update November 20, 2009: According to How long have you been tweeting, my first Tweet was on March 16, 2007. Feels like it should be longer than that.