Rita’s Italian Ice, a Pennsylvania-based franchise with 560 stores in 18 states, launched a Facebook promotion/contest on March 20 that has since helped the company add almost 100,000 fans, with 58,500 coming in the last week — landing the page on at 17 on our weekly list of top 20 growing Facebook Pages.

The company’s PEEPS Fandemonium Facebook Contest, running through April 4, has helped the Page reach 201,400 fans. Partnering with the makers of marshmallow candies PEEPS, Rita’s is awarding a year’s supply of free ice to a fan a day until the end of the contest, as well as a free gift bag for every 1,000 fans that includes free ice coupons, a tote bag, hat and t-shirts.

Rita’s Director of Corporate Communications Kelly Banaszak tells us that, although the Facebook Page was created a year ago, the company’s fan base didn’t “explode” until a free year’s supply of Italian ice was on the table. Combining the launch of the Fandemonium contest with a watershed annual event for Rita’s — the first day of spring, March 20, when the company gives away 1.4 million free cups of ice — Banaszak says Rita’s was able to encourage more than 1 million actual customers to visit its Facebook Page.

Each of the 1.4 million customers were told on March 20 about the Facebook contest upon receiving their free ice.

“We created the promotion to encourage customers to go online and become a fan of Rita’s on Facebook because we have a lot of avid fans of the brand, we wanted to give them an opportunity to interact with our brand online —to extend that store experience,” Banaszak tells us.

Entering the contest requires users to visit the PEEPS Ice tab where they enter name, email and opt-in to the contest, then have the chance to publish this fact to their Wall; they may also choose their “PEEPS-o-nality” from several PEEPS caricatures, and also publish this to their Wall, giving the company two chances to spread the word about the contest via its entrants. For those who want more information the WNMR (World Needs More Rita’s) tab helps users find locations, merchandise and find information about other promotions.

Banaszak says the company wanted to extend that success to Facebook both because it’s a popular site, and also because Rita’s has been focusing its marketing resources on social networking sites like Facebook, as well as Twitter and YouTube. The company even hired a social networking coordinator last year to better manage the company’s new outreach efforts.

We’ll be tracking how the campaign goes through the 4th. For more on using sweepstakes as part of a campaign, check out our Facebook Marketing Bible.