Twitter : Visitor statistics from public sources

I have compiled visitor statistics for Twitter.com from a few public sources.

Quantcast
Twitter.com : rank 38,104
Estimated Monthly Unique Visitors from the US : 46,688

Alexa
Traffic Rank for Twitter.com : worldwide : 595
Traffic Rank for Twitter.com : US : 285

Compete
The number of unique visitors : 122,112 (05/2007)

Ranking.com
Web rank : 3,649


Looking at the numbers and the different screenshots, one can wonder if these statistics are all for the same site (yes they are ...).

Public source statistics must be handled with care. Can we trust the data they are reporting ? May we consider the data to be complete and correct ? Different tools give different results. Interpretation of the quantative data is not easy, as each site has its own vocabulary. While looking for alternatives for Alexa, Compete, Quantcast and Ranking.com, I came accross rather disappointing news.

It appears that the external metrics available for competitive intelligence on the web today simply do not provide a significant source of value. ... Services like Alexa, Ranking.com, Compete.com & Netcraft are nearly useless when it comes to predicting traffic or comparing relative levels of popularity, even when used on a highly comparable set of sites in a similar field. ... The sad conclusion is that right now, no publicly available competitive analysis tool we're aware of provides solid value. Let's hope the next few years provide better data.

Conclusion
I have to be very careful with my interpretation of the graphs from the different sites. The three graphs do show the same trends. I think it is save to say that the sharp increase of traffic towards Twitter.com is over. There seems to be a stabilisation from April 2007. The level at which the traffic stabilises is difficult to determine.