That Amazon would sell a fair share of Kindle Fires seemed inevitable based on the estimated volume of pre-orders the world’s largest online retailer had received before its tablet device launched.

But hard numbers are starting to trickle in now that the Fire is actually shipping, and those numbers are impressive.

According to market research firm iSuppli, the Fire, which was the best-selling tablet on both the and websites on Black Friday, will reach 3.9m in unit orders this quarter, making it the second most popular tablet in the world.

iSuppli’s numbers jive with a report published by DigiTimes, which says that shipments of the Fire from manufacturer Quanta Computer have reached three to four million units thus far. What’s more: according to DigiTimes’ sources, shipments could reach five million units by the end of January or sooner.

Amazon obviously picked a good time to launch the Fire (right before the holiday shopping season), and it appears that the Kindle’s price is also a big factor. At $199, the tablet device is in a sweet spot, and Amazon’s Dave Limp revealed that the company is “seeing a lot of customers buying multiple Kindles — one for themselves and others as gifts.

The big question now for Amazon is whether it can keep the Kindle Fire momentum going past the holidays. If Amazon can convince new Fire owners who don’t have an Amazon Prime membership to shell out an additional $79 for one so that they can take advantage of e-book lending features and free streaming movies, Amazon may very well be able to build an iPad-like customer base. Which will be extremely helpful when the company unveils a second-generation Fire.

Once again, Apple should be concerned. Although it’s too early to know how many iPad sales the Kindle Fire will displace, it does appear that the first experience many consumers will have with a tablet will come by way of the device, and thanks to Amazon’s ecosystem, many may never look to ‘upgrade‘ to an iPad.