When you provide an RSS feed on your site you are allowing your site visitors to subscribe to your sites content and other sites to syndicate your content. Every time someone subscribes they are in effect voting for your site.
RSS feeds are a very democratic tool. A subscriber has complete control over whether or not they subscribe in the first place and whether or not they stay subscribed. This makes RSS a good measure of the ongoing popularity and quality of a site over time as subscriber numbers can go up and down.
RSS as a metric
Proving your RSS subscriber base is more difficult, luckily there are third party tools such as Feedburner that can provide these statistics for you. Having an impartial third party measure your RSS subscriber base makes your numbers more trusted.
Increasingly advertising agencies are starting to use RSS subscribers as a metric to decide the value of advertising on a site. ReviewME uses the number of RSS subscribers to a site as part of its metric measurements and converts this into a star rating.
Another site using RSS is URLfan that is using RSS feeds to find out which websites are being referenced most often. As more and more internet users become RSS aware it will become more widely used metric for measuring a site's credibility.
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