For the first time ever, U.S. mobile operators reported a net decline in postpaid subscribers, who sign up for ongoing service and pay a monthly bill, according to a report released Monday by Chetan Sharma Consulting. The seven major carriers together lost about 52,000 postpaid subscribers from 2011's fourth quarter to the first quarter of 2012, the report said.

The drop may have been partly seasonal because the first quarter is typically slow for postpaid subscriptions, said analyst Chetan Sharma, who wrote the report. But he cited two other reasons for the trend: Consumers have been shifting to prepaid, no-commitment deals because of economic hardships, and the carriers may have tapped out the supply of new mobile subscribers who can commit to a monthly bill. Sharma defines postpaid services broadly, as any plan in which a user pays monthly rather than buying a certain amount of service and "topping up" that supply when it's used up. Former postpaid subscribers may or may not come back when their fortunes improve, Sharma said.

Carriers have spent the past two decades bringing more U.S. consumers into the mobile world and competing among themselves for those new mobile users. But the U.S. market now has mobile penetration of 110 percent, if connections to non-phone devices such as tablets and industrial equipment are included. Most growth is now coming from non-traditional devices and revenue sources, Sharma said. Key among those are connected devices, which include tablets, laptops and personal hotspots, as well as industrial gear such as electric meters.

The U.S. mobile operators had no trouble racking up revenue gains in the quarter. For the top seven carriers, revenue grew 6 percent from the previous quarter and 21 percent from a year earlier, reaching US$18.7 billion, Sharma reported. Verizon and AT&T achieved the highest mobile carrier revenue in the world as AT&T jumped ahead of Japan's NTT DoCoMo to enter second place, according to the report.

Connected devices led year-over-year growth in the first quarter, rising 23 percent. Revenue from the major carriers' own prepaid services rose 15 percent, while money coming in from wholesale deals, primarily third-party prepaid carriers, increased 10 percent. Postpaid revenue growth was just 1 percent, Sharma reported. Many postpaid customers have left for prepaid plans, which don't require a long-term commitment, and they may never return, he said. To keep making more money from their postpaid users, which still make up about 66 percent of all customers, the operators are selling phone and service upgrades and getting non-data users to adopt mobile data plans.