This post was inspired by a Reddit admin's comment showing that Quantcast was far off: http://www.reddit.com/r/reddit.com/comments/cahpw/digg_is_dead/c0r9904
Please post numbers for months that are fully shown at all those sites already, so not from this month. You don't have to post your domains for privacy reasons, but if you would, please do.
If you're not quantified, Quantcast is a joke and the other sites are always a joke, no matter what. There was a long time when Alexa thought the top 20 sites in the world were all in Korea because, I guess, everybody in Korea had the Alexa toolbar. One day they changed their formula to prevent this because people were pointing out that their stats make no sense.
Compete, in particular, has a free service that's designed to annoy you into paying for the service, but everything I see on their free service and on their blog convinces me that the paid service is worthless... For instance, they had nothing to blog about one day, so they blogged that "Girls Gone Wild" gets 30% of its traffic from women... Everybody knows that systems like that use 50-50 gender statistics as a bayesian prior, so they always claim that sites have more balanced audiences than they really do.
Alexa is comically low on pageviews and maybe 6x low on reach when we compare those stats to other sites who make their numbers public. However, Alexa tends to head in the right direction.
I looked at Comscore data on us a few years ago and it was less wrong than Quantcast and Compete, but still well off the mark.
As an example, I once spent a year browsing with the Alexa toolbar, and it boosted every one of my sites up into the top 100k just from my own traffic. After ditching the toolbar, they all dropped back down.
As to accuracy, Compete isn't even on the right order of magnitude for any of my sites. It's way over for one, way under for another. Just no correlation to reality.
Google Analytics (unique visits): 77K
Quantcast: No Data
I really have no idea how to find comparable stats on Alexa (since it uses percentages) but you can check it to see.
Compete's referral data and search analytics were also way off. I might think this is due to the site being pretty new ... but I'd rather they said nothing (like Quantcast) than be way off.
Based on how wrong they are for my site, I would not put much stock in them for other people's...
I use Compete stats as relative measurements against other websites (assuming everyone's off by a similar ratio). I use Quantcast stats to look at the demo info.
I originally took the Quantcast code off my site b/c it slowed things down and despite having fairly accurate counts, it had skewed demographics. I didn't want to encourage them, so I took off the script. However, my use of IntenseDebate meant that I was still using it.
I don't think IntenseDebate is doing anything wrong, but I would like an opt-out.
Month: May '10 (May 1 - May 31)
Google Analytics: 307,000 "Visitors/Month" (Unique Visits)
Alexa: 25,856 Traffic Rank (has been as high as 2,000)
Compete: 72k "Unique Visitors"
Quantcast: 201.5k 'Worldwide Visitors'
Comscore: 144k "US Visitors" (we have to pay more than $30k/yr for global access)
Google Analytics: ~1,000,000 visits/month
Compete: 110,000 visits/month
Alexa: 9,548 traffic rank?
Quantcast: 5,000 visits/month
They all seem to be very low and very all over the road.
Google Analytics - 470k uniques
Compete - 124k
Quantcast - 156k (not "quantified")
-----Rank (at present)
Alexa - 19,730 in the US
Compete - 16,735 (US?)
Quantcast - 12,957 in the US
Site #1 (classic gaming related)
April 2010 - Unique Visitors:
My server logs: 16000
Site #2 (music related and random things)
My server logs: 5000
Compete.com is US only traffic so it is pretty spot on for your second site. However it's weird that your first site with 500+ uniques/day avg. doesn't have a quantcast estimate.