When Facebook co-founder Mark Zuckerberg recently claimed that more than half a billion people now regularly use the social networking website to “stay connected with their friends and the people around them,” he wasn’t just counting herding teenagers and partying millennials.

Delta Air Lines (NYSE:DAL) jumped into the fray Thursday  with the launch of what it says is the travel industry’s first e-commerce capability on Facebook, allowing users to book flights with their friends directly on Facebook.

In a little more than six years, Facebook has become the world’s third-most-populous “country,” behind China and India.

It also has become a serious business tool as companies, nonprofits, industry associations, governments, political candidates and parties attempt to reach audiences important to them.

Zuckerberg’s blog pointed to three “Facebooks stories” that recount the role the networking site played in restoring a community playhouse in Kentucky, gathering 100 jogging partners for the Danish prime minister and supporting breast cancer patients undergoing  treatment.

The popular social media site can also bite back, as Target Corp (NYSE:TGT) is learning. Its Facebook page has been transformed into a debate forum on sex and politics following the controversy arising from the company’s $150,000 contribution to MN Forward, which is supporting Tom Emmer for governor.

Both BestBuy and Securian Financial Group also made contributions to MN Forward but have not received the sort of heat Target has, either in the news media or on their Facebook pages.

Speaking of Securian, the St. Paul-based financial services company launched its Facebook page last month, having taken a trial social-media run as sponsor of the Securian Frozen 5K and Half Marathon during the St. Paul Winter Carnival.  Securian  set up a Facebook page that allowed runners to exchange weather- and racing-related information in advance of the meet. The page is still up, allowing runners to discuss next winter’s race.

The new corporate Facebook page has a different role for Securian. Because it sells financial products that are highly regulated, the page “is not [intended] to market or sell our products,” said Securian spokesperson Maggie Jensen. Instead, it will talk about recruiting efforts, employee involvement and employer awards to build awareness of the company’s reputation “as a great place to work,” she said. The company will use Facebook to help recruit potential employees and grow its network of independent financial advisers. “We have lots to talk about,” Jensen observed.

Shortly after Facebook crossed its half-billion milestone, Mayo Clinic announced it was launching the Mayo Center for Social Media to accelerate adoption of social media for health-related purposes and, by extension, improve health literacy, health care delivery and population health worldwide.

“We are excited about the impact [social media] can make in health care,” said Lee Aase, Mayo’s manager of syndication and social media, saying that the venture has “top executive-level support” within the organization. Pointing out that it can take up to 17 years on average before new medical knowledge and practices are widely accepted by patients and the larger medical community, Aase said, “If we can cut a few years off that, it could be huge.”

Mayo Clinic jumped into online media in 2005, when it began podcasting interviews with staff physicians. The podcasts captured page one billing on iTunes and became a hit, Aase said. They also launched a Twitter feed and Facebook page early on, originally to prevent someone else from claiming the Mayo name, a practice called “brand jacking,” he explained.

With blogs, a popular YouTube channel, 60,000 followers on Twitter and 20,000 on Facebook, Mayo has become a leading online social media source in the health care field.

Other hospitals and clinics began seeking them out for advice. “We want to encourage [the use of social media] but there is only so much free advice we can give,” he said.  Mayo will consult with other health care organizations through the center “to at least cover our costs,” Aase explained.

“Facebook is growing in massive leaps and bounds in the 30, 40, 50 age group, particularly among women,” observed online marketing executive Erin Rauk, co-CEO of BingoTheory.com in Minneapolis. “In those older age groups, it’s growing like crazy.” She also said it “makes a lot of sense” for companies to use Facebook as a human relations department retention and recruitment tool.

Mykola Sarazhynskyy, a University of St. Thomas marketing professor, said that companies using social media in traditional, uni-directional communication are missing the point of the medium. Instead, he urges companies to seek “meaningful engagement” with their intended audiences.

He also sees social media as a way to build brand awareness and loyalty longer term.  If a company’s customers are 40 or older, for example, a company can use social media to build its presence and brand awareness today with younger people who will become future customers.