Based on a running tally on its site, ad firm Chitika Labs estimated that Apple has sold more than 562,000 iPads since the tablet computer went on sale on Saturday.

The tally is, if anything, a conservative measure. It’s based on the number of new iPads that are accessing the Internet via the device’s Safari Web browser and unique hits to Chitika’s ad network. In a post, Chitika research director Dan Ruby said the numbers showing up in his company’s data are significantly lower than the official and estimated sales. By the middle of the day on Monday, Chitika had logged about 270,000 iPads on its network, even as Apple had said it had already sold more than 300,000 units on Saturday alone. The discrepancy is possibly because some users may have gone online but not yet browsed Chitika-served websites.

Of course, there are lots of things for iPad users to do with their iPads besides surf the Web. They can download and play games, read electronic books, watch movies, listen to music, and so on. Those activities wouldn’t show up in Chitika’s count. One sure conclusion: Apple’s continuing to move iPad units.