A quick search on google will let you know how completely unreliable alexa rankings are. ...BUT advertisers DO use them...which is tough.
eh what the hell here is one link
and a quote from alexa
"This means that, for example, the difference in traffic between a site ranked 400,000 and a site ranked 800,000 is statistically negligible. Sites ranked 100,000+ are subject to large ranking swings due to the scarcity of data for those sites. In such cases, a single user (or the lack of one) can make a dramatic impact.
" -
source
and as an ex-research mathematician/ math prof, I can tell you that yes, their method of stats is very very bad lol. Thats my opinion
....to the original poster. I simply think you dont yet have enough visitors to profit from advertising, or sell it easliy. Good luck though and keep at it, as I think its a great idea and will eventually be at the point where advertising makes sense.
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edited to add **you can also find that disclaimer about rankings on alexa's site itself although its buried
here