Now that the use of RSS aggregators is really starting to take off, people are peering into the question of market share. Which aggregator is the favorite among users — and thus best positioned to generate a viable revenue stream? Some data is starting to emerge, though a lot of it is seriously back-of-the-envelope kind of stuff.

A couple of weeks ago, the Pew Internet & American Life Project released a report estimating that “5 percent of internet users say they use RSS aggregators or XML readers.” That’s about six million Internet users, and the number is probably higher because Pew only surveyed adults. Then we have this posting from the folks over at FeedBurner, a service that manages feeds for publishers. They found, looking at the top 800 feeds they manage, that Bloglines users accounted for 32.86 percent of traffic (there are lots of caveats about the accuracy of the data, so take it with a grain of salt). That’s far ahead of anyone else (NetNewsWire was second). And by our math, that gives Bloglines, based here in Woodside, nearly two million users.

That would seem to put CEO Mark Fletcher in a pretty enviable position once he finally starts “integrating highly targeted contextual advertising” into the service later this year.

Why is Bloglines so popular? Aside from being reliable and easy-to-use, our guess is that it has something to do with what Paul Graham said about Web-based applications in his recent book, Hackers and Painters. “The idea of ‘your computer’ is going away and being replaced by ‘your data.’ You should be able to get at your data from any computer. Or rather any client, and a client doesn’t have to be your computer.” As a Web-based application, Bloglines offers that convenience.

Note: Also interesting is that two of the top RSS feed readers in the FeedBurner list are for podcasting. Look how fast that trend’s growing.