After signing up only 106,000 in the first month of operation, the online marketplaces selling individual insurance plans through the Affordable Care Act have dramatically increased enrollment, with about 2.2 million people through December.

The release of the third month of data from the Department of Health and Human Services marked a couple of firsts: the 36 exchanges run by the federal government now have far more people signed up for coverage, and the data includes more detailed demographic information.

The online marketplaces sell insurance policies to those who don't have affordable coverage through their employers, most often with some financial assistance. Some 79 percent receive subsidies that reduce the cost of premiums. Thirty-six states chose not to host their own marketplaces; the other 14 and the District of Columbia created their own websites.

Through November the states that created their own exchanges were outpacing those relying on the federal government. But through December about 1.2 million people signed up for coverage through the federal exchange while close to one million selected plans through state-based marketplaces.

Enrollment figures provided by HHS include those who have selected a plan but not necessarily paid the first month's premium. Twenty-four percent of the 2.2 million are between the ages of 18 and 34, which is considered by health experts to be an important age group for keeping premiums low. Younger people generally come with fewer health problems and help even out the costs of older and sicker people in the insurance pool. The Obama administration set a target of 2.7 million in the 18-to-24 demographic out of 7 million expected to sign up for coverage in 2014, which would be about 38 percent of the total.

Although signing up "young invincibles" is considered crucial, the law includes programs designed to stabilize premiums if not enough healthy people sign up for coverage.

In the states that are running their own exchanges--all of which are expanding Medicaid to more low-income adults as part of the new health law--about 833,000 are eligible for Medicaid or the Children's Health Insurance Program. About 751,000 are eligible in the 36 other states, most of whom are not expanding the program. That means a significant portion of those people were eligible under existing Medicaid rules but weren't aware, and they'll be covered at existing federal matching rates instead of the level for the expansion population, which is at least 90 percent for the duration of the program.

See the full HHS report here.