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Facebook may seem some days like a laundry list of "just married" profile updates complete with images of smiling brides and grooms, but according to the Pew Research Center, barely half of U.S. adults are married, the lowest percentage ever.

W. Bradford Wilcox, the director of the National Marriage Project at the University of Virginia,  said that marriage had been "in retreat" in the last 40 years and that the decline had accelerated since the recession started in 2008.

"Marriage is less likely to anchor the adult life course," he told ABC News today. "It's less likely to ground children's experience with family life. It plays a less central role as an institution in American life."

In 1960, 72 percent of U.S. adults age 18 and older were married compared with 51 percent today. The median age when adults decide to finally take that big step is also the highest its ever been for both men and women - 26.5 and 28.7 respectively.

The most dramatic decline in marriage occurred among those 18-29. Just 20 percent of them are now married; 59 percent were married in 1960.

Wilcox said that people felt more comfortable postponing marriage until their late 20s and early 30s these days. He said the 20s were viewed as the "odyssey years," and a time to "find yourself."

For many, Wilcox added, marriage is still viewed as an economic institution, not just about love and living happily ever after.

"People are looking for a soul mate but also a person with a decent job," he said today. "The bar has been raised.  Expectations are higher."

Pew, which examined U.S. Census data, said that other living arrangements - including cohabitation, single-person households and single-parents households -  were becoming more prevalent. The number of new marriages fell by 5 percent between 2009 and 2010.

Wilcox said that while U.S. adults without college degrees were marrying less, they increasingly were having children in nonmarital situations.

"In the minds of Americans, getting married and becoming parents are two different things," he said. "Their top priority is being a parent, second to having a successful marriage. People have separated the two things. Years ago, they were closely linked to one another."

"The bottom line is that kids are experiencing more instability and more hardship because the adults are less likely to get and stay married," Wilcox said.

Seventy-two percent of U.S. adults had been married at least once, though this was a decrease from 85 percent in 1960.

A survey done by Pew and Time magazine in 2010 of 2,691 Americans found that nearly four in 10 Americans said that marriage was becoming obsolete. Forty-four percent of those 18-20 said it was obsolete.