Alexa rank is depend upon the traffic that recieves to the site. Alexa provides the complete details about a site which includes the traffic that recieves to a site daily, daily ads revenue, the worth of the site and many more. Some bloggers has a thought that based on the alexa rank they will get more visitors which is totally wrong, based on the traffic that the alexa increase. Having a good pagerank and seo score is the only way that one can increase there traffic and alexa rank. Some webmasters use a dirty trick called traffic exchanging systems to increase the traffic and alexa rank it is fully foolishness so concentrate on increasing traffic on straight way. Building of quality backlinks and publishing of trendy contents will help a site to get good pagerank and seo score in that way they can increase the traffic. There is no need to look on the alexa rank just only concentrate on the pagerank and seo score. Alexa rank is given according to the traffic that a site got in the last 3 months based on that they will list the site in the order of the audience. The alexa rank is updated daily based on the rotation of days of the month. Alexa has a good pagerank so adding the site in the alexa is way of getting a strong backlink for the site. The alexa also help to create a competeting mind among the webmasters to increase their site rank.
There are thousands of sites in the internet which doesn't have an alexa rank but the site is added in it, it is because most of the sites in alexa are not claimed or otherwise it doesn't have much traffic. Having an alexa will be very useful when it will show to others in different site review building sites. So when doing blogging only look in increasing quality traffic from there the alexa rank will increased automatically. All the best.