So what did you do on your holiday weekend?

Robyn Mendenhall Gardner of Kalispell, Mont. was planning a relaxing few days with her husband and eight children before a business trip this week. But Ms. Gardner, who recently turned 40, has dedicated herself to fitness in recent years. So before bed on Friday night, she logged onto Facebook to create a fitness-related challenge for herself and a few friends.

"Honestly, I found it online," admits Gardner. "It's not really my own. I'd tried this challenge myself, but I quickly lost interest. So I decided to invite some friends and family members to keep me motivated."

Fast-forward to Tuesday morning, when Robyn awoke to find that the "30 Day Ab Challenge for those who need some motivation" had attracted almost 600,000 people across Facebook. By lunchtime, the number was almost 850,000.

"It's a little overwhelming, but very exciting," says Robyn. "I was hoping to motivate others—and therefore, motivate myself—but this is beyond anything I could have expected."

The explosion of social media over the past decade is often lamented, but the 30-Day Challenge is a perfect example of the power of sites like Facebook. Gardner works as an independent ambassador for Plexus Worldwide—whose products she credits for helping her lose almost 60 pounds over a nine-month period. However, the 30-Day Challenge isn't sponsored by any company—rather, it's a testament to the idea of motivation among people with similar goals.

"We went camping over the weekend," says Robyn, "and I got a message from a friend who told me 'Your Facebook is blowing up!'"

"I know other events like this have been created—I've seen them," she continues. "I can't explain it, but this one went viral."

The 30-Day Challenge isn't too complicated—beginning on June 1, participants will complete daily series of workouts and exercises targeting the abdominals. Sit-ups, crunches, leg raises and planks are the movements at the heart of the challenge, occurring in increasing numbers up until June 30. The challenge follows a strict three-days-on, one-day-off schedule for the balance of June.

If you're interested in joining up with the wildly popular challenge, the host page can be found at Robyn posts on the page herself, and participants are invited to share photos or videos of themselves completing the challenge or 'before-and-after' type photos of themselves. Along those lines, Robyn Gardner pointed out something for the hundreds of thousands of followers she's gained.

"No, those are not my abs at the top of the page," she says. "People should know I did not start this challenge with rock-hard abs. That's my motivation picture—every time I'm eating something I shouldn't be, that picture reminds me of where I want to be."

Read more Sports Doc for Sports Medicine and Fitness.