Sites like Alexa and Compete provide valuable data - you just need to know how to use it.
The number 1 thing you need to remember is that the data is not useful as an absolute measure - it's only good to compare sites.
The number 2 thing to know is that if you compare similar sites, the comparisons are more accurate. So if you compare trailbehind.com, everytrail.com, and backpacker.com, the comparison will be a lot more accurate than if you compare cnn.com, stackoverflow.com, and myspace.com.
Number 3 is these sites are more useful if you have benchmark data. For example, if you want to know the traffic for a magazine type site, go download their media kit (usually available as a PDF on their site), and it will tell you their traffic. Then you can use this data for understanding Alexa better.
I'm honestly not trying to attack you but there are several posts below (mine included) that indicate what you are saying isn't the case. If you have a factual basis that says we're wrong I'd be interested in hearing it. But in this post you are simply stating absolutes without backing those absolutes up.
I guess what I'm asking is "how do you know what you are saying is true?"
Go to InformationWeek.com, ComputerWorld.com, and ITWeek.com, or pick a few other trade rags that publish a media kit.
Download the media kits, see how much traffic these similar sites get, and then see what the error is on Alexa. I think you'll find that the numbers are of the right order of magnitude.
You're going to get a pretty clear signal when looking at sites relative to one another, since most of the error will effect the sites the same. And if you have sites that have the same sorts of traffic patterns, then the relative signal should be even clearer.
It's true that people gaming the system effect the numbers, but that's not going to distort the numbers enough to make them worthless.
You can't use Alexa or similar to see whether one site has 3000 visitors a month and another has 3500. But if one ha 3000 and one has 30000 then you'll see it in the graph.
Sorry about the name typo (<pre coffee excuse>). Although the absolute numbers are always wildly inaccurate, Compete tends to be fairly good at viewing the traffic trends for a site. Or does the chart look nothing like your own internal ones?
I'm not even sure that's true anymore. Compete's always low-balled my traffic numbers (at least in comparison to Google Analytics) but I have a fairly small blog so I've always given Compete a pass because I can understand why it would be harder for them to measure a site with only a modest readership.
But a couple months back I got linked to by Mathew Ingram and, while Michael Arrington was covering Mr. Ingram's post on Techcrunch he also shot me a link. As you can imagine that generated a huge bump. But Compete actually had my numbers trending down for that month (cut in half in fact) and the month after as well (which was completely contrary to my Analytics readings).
So now I don't even have much faith in the site for trending data.
Interesting situation... Should the title's "Spolksy" be changed to "Spolsky"? If it isn't changed, the title is inaccurate. If it is changed, your comment won't make sense to new viewers anymore. (Except now your comment would still make sense, because of this comment! Fun stuff.)
Care to tell us how you managed to get that first wave of traffic? I'm guessing that having a popular blog helps, but who were the first adopters, and how did you get them?
Being someone that got in fairly early (3-digit SO ID#), in my case it came from listening to the podcast. Months before the site went live, Joel and Jeff discussed the site and how it was being built.
You are right, though; the reason why I started listening to the podcast was because I enjoyed reading Joel on Software and Coding Horror, and I've continued to stay because the show is much more casual than any other programming podcast I've come across (other shows I've found tend to take themselves a little too seriously).
Anyways, that's just my opinion, but I do think that a podcast is a great way to build buzz for a site, and if you know of any shows similar to Stack Overflow I'd love to hear about them.
You cant use compete.com as a source by itself. It is best used to compare. So if it is always off by x% (or a range of %), you can still have a rough comparison to other sites.