Two of the nation’s biggest department stores — Macy’s and Sears — are using social media to drive prom sales for the first time this year.

Sears this month launched the Ultimate Prom Experience, a microsite dedicated to helping teens find the perfect dress. The site, accessible at Sears.com/prom, includes features such as a “Find out your prom [dress] personality” quiz, hair and makeup tips, a list of the 10 hottest trends, and a $1,000 sweepstakes. Quiz takers may also share and post the results on Facebook or Twitter.

Macy’s, meanwhile, launched a Prom 2010 tab on its Facebook fan page. (It currently has more than 377,600 fans.) The Cincinnati-based department store chain will post prom photos submitted by consumers on Facebook. It also has set up a Twitter hashtag, #prom2010, so consumers can tweet about in-store prom events. As part of a partnership with Teen Vogue, Macy’s is hosting “prom parties” at its stores through May. The retailer will provide VIP treatment to the first 75 prom shoppers that text a special promotional code at select, in-store events, said Macy’s rep Orlando Veras.

The move is a departure from the typical print, e-mail and direct marketing campaigns retailers run around prom time. It also reflects the growing power of social and mobile media marketing, particularly after last year’s Black Friday sales, said Lippe Taylor president Jim Joseph, whose PR and communications firm has worked with companies like David’s Bridal on social media, prom-season campaigns.

But Macy’s and Sears aren’t the first retailers to use social media to drive prom sales. For the past three years, Charlotte Russe, for instance, has been using a tool called ShopTogether, to help teens in their prom dress search. It licenses ShopTogether — which lets two, three or more users shop, chat and browse online picks, similar to a chat room — through social media marketing firm DecisionStep, which also works with brands like Mattel and Foot Locker. DecisionStep CEO John Jackson said since using ShopTogether, Charlotte Russe has seen an increase in conversion rates, order sizes and time spent on the site. Even more important, shoppers are bringing in new visitors, including those who have never heard of the brand, Jackson said.

The challenge for many retailers is linking brand engagement — and exposure — with sales, said Sucharita Mulpuru, a Forrester analyst whose area of expertise is social commerce. “No matter how many fans you get to a Facebook page or how many Twitter followers you get, it’s not nearly the same home run from an integrated marketing standpoint compared to things like paid search or e-mail marketing, or even search engine optimization,” Mulpuru said.

Marketers, too, must be cautious about the types of social media channels they employ. “A lot of marketers lump the [different] social media outlets together,” said Joseph. In the case of teens, particularly, Facebook may be a more effective platform for reaching prom-scouting consumers, versus Twitter, which has evolved to be more of a “business-y space” for sharing and communicating information, he said.

Sears this year decided to reach out to teens via social media in order to “make a more personal connection with [them],” Sears rep Tom Aiello said.. But the retailer isn’t pinning all its hopes on social media. Sears is doing blogger outreach, and it’s also targeting prom shoppers via e-mail and advertising. And, just like in past years, Sears is courting fashion and beauty editors as part of its print magazine outreach.