Feedburner is now owned by Google Inc. It is most popular RSS feed delivery application. But there are many other methods to read RSS feeds and many people use alternative methods as well. Using feedburner you can show the number of subscribers you have. But feedburner counts only him who is using feedburner to subscribe your feed.
So using FD Feedburner plugin you can redirect you all other subscribers to feedburner and this way feedburner will count your all subscribers as feedburner subscribers.
To use this plugin you need WordPress 2.0 or higher, I do not know it is compatible with WordPress 3.0 or not.