Building up a robust social network should be as much of a retirement-planning goal as building up a robust 401(k). "Social networks are incredibly important and become meaningfully more important as we age," says Jacob Brown, a therapist in San Francisco.

Deborah Heiser, an applied developmental psychologist in New York, says research shows that people who have even one or two close pals live longer and have more emotionally stable lives, so people should not worry about building a phalanx of friends. "It's not the quantity that matters," she says. "It's the quality."

Knowing what you want in friends can inform where you look for a quality match. If public service is important to you, a charity like Habitat for Humanity would be a good place to start. Following are nine popular options for people approaching or in retirement.

Help a charity. Several websites help people find volunteer opportunities. Some, such as the Aging Mastery Program and Senior Corps, place retirees. Taproot matches people who have specific skills with nonprofits that need them. Volunteer Match has a large selection and a good geographic filter. Idealist.org includes events that need one-time volunteers.

Share a hobby. Whether you are a canasta fanatic, hiking enthusiast, wine lover, swing dancer, karaoke singer or soccer fan, you can find like-minded people near you via Meetup.com.

Take a class. Adult education offered by local school districts can help you find people who share your desire to learn about fine art, technology, languages, cooking, literature or even comedy. Many universities permit older adults to monitor courses at little or no cost. The Osher Lifelong Learning Institute directory and state departments of aging have details.

Move around. Following doctors' orders to exercise three days a week can help you meet people. Whether you prefer bowling, cycling, golf or tai chi, invite a neighbor or acquaintance to join you. Alternatively, see if the local parks and recreation department offers recreational team sports such as softball, basketball or pickleball.

Walk the dog. Walking with purpose is a good way to exercise; walking with your dog is a good way to socialize. While pets are not props, your pooch can be a cute icebreaker. Several studies have shown that people are much more likely to approach a stranger who is walking or playing with a dog than a stranger alone. The American Kennel Club offers advice on finding — or starting — a dog-walking group in your neighborhood.

Keep the faith. Houses of worship often host regular activities for their congregants. Many schedule events specifically for older members.

Leverage social media. Social media sites such as Instagram, LinkedIn and Facebook make it easier to find old friends and renew relationships.

Get a job. Whether or not you need a paycheck to supplement your retirement savings, getting a job outside your home for a few hours a day or week makes it easier to meet people — customers as well as co-workers.

Turn a new page. Ask the public library or neighborhood bookstores if they can recommend book clubs focused on particular genres, authors, topics or time periods you like.

It helps to pursue an activity you like, whether that is cycling, bird-watching, swimming or being a docent at a local museum or historical society.

If you give it time, something will come to you. "Once that clicks," Heiser says, "things fall into place."

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