According to a recent report:

• In 2012, 15% of individuals reported they contributed nothing to their Health Savings Account (HSA)
• An additional 7 million reported that they were covered by an HSA-eligible plan, but had not opened such an account.
• Altogether, 18.6 million adults ages 21–64 with private insurance, or 15.4% of that market, were either in a consumer-directed health plan (CDHP) or an HSA-eligible plan, but had not opened the account that would be used to fund covered expenses.

Source: "HRA/HSA Health Plan Contributions Continue to Grow," Employee Benefit Research Institute (EBRI), February 20, 2013, http://www.ebri.org/publications/prel/