AT&T reported its fourth quarter results today, telling the world that it sold a record-breaking 10.2 million smartphones, most of them iPhones. And while the company is still posting a massive loss this quarter due to Superstorm Sandy and unexpected pension costs, it was generally a good quarter, with $32.6 billion in revenue.

One thing that’s almost too good is AT&T’s mobile share plans, in which a subscriber can buy a single data plan for multiple devices, such as a laptop, phone, and tablet.

The plans are superpopular, AT&T said, with 10 percent of its customers, or 6.6 million people, having signed up. Users average three devices each, and a quarter of them have purchased plans with 10GB/month or higher, all of which add up to a much higher uptake than AT&T anticipated.

That was a bit of a problem for the company, as it reported, saying that “data revenue growth was slowed somewhat by the growth of Mobile Share plans.” In addition, sales of tethering plans and mobile Internet devices such as MiFi’s were also down due to the share plans.

Which is probably more shocking to cellular subscribers than anything else: a massive telecom actually did something pretty customer-friendly, even at the cost of some extra revenue.

AT&T did not break out how much it lost due to sharing plans. Nor did it speculate on how many customers the carrier might have lost if it did not offer the customer-friendly option.

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