PageRank (0-10) has nothing to do with the ranking of your website.

Linkbuilding typically increases my pagerank, I've found.

I have no idea what connection you are trying to make between the ethics of linkbuilding and how pagerank works though.

Great question KingTaw,

Keep in mind that Page Rank is just one of many factors that Google uses to assess the importance of a website. Google relies heavily on other websites linking to one another as indicators of the popularity of pages so they can deliver the best results to users. Some pages may be really well optimized and appear high in SERP’s yet have a low page rank. In contrast, some websites may have a high page rank yet they may be poorly designed and have less than interesting content.

Check out this comprehensive post talking about page rank,

What Is Google PageRank? A Guide For Searchers & Webmasters

See also this post that talks about Page Rank in relation to SERP’s,

Page Rank vs SERP

Hope that gives you some insight,

Shawn

Ethically? Yes I believe it's fair to build backlinks.

Alexa is not even a real rating...it's results are tabulated from people who have the toolbar installed and then figured on some multiplier to estimate the traffic.

Your site's PageRank has no direct bearing on search result placement. The PageRank of your backlinks DOES have a direct bearing on search result placement.

I don't really see the problem here. If your website is already generating 100.000 page views/day, it means you are getting traffic, so why bother about PR? And about the Alexa ranking, that's not really a great indicator for a website popularity. Alexa ranks websites in terms of web surfers who have the Alexa toolbar installed and visit the site and besides people that work in the internet marketing business, whether SEO, Social Media Marketing, or any other aspect, I don't think many people have it installed.