Opinion on Twitter is divided. What seems to be undisputed is that right now it's growing very quickly. I was curious just how "quickly" quickly was, preferably going beyond just anecdotal "my network doubled in size in the last 5 seconds" kind of observations. It seems like Twitter assigns globally unique, incrementing IDs to all messages it receives. By looking at the values of these IDs over time, it's possible to see how many status messages Twitter is keeping track of. I've generated a logarithmic graph of this.
I'm not sure why there was an inflection point in early November. It's also possible that this is affected by technical changes on Twitter's side. Still possibly interesting. Also, Joshua's post on autoincrement considered harmful is related and an interesting read.
Update: As it was pointed out to me in the comments, Andy Baio had the same idea except he executed it more throughly.