The social web used to be about accumulating fans. Now it's about building engagement.

OK, but how do you do this most effectively?

By constantly experiment, advised Susan Lyne, who chairs one of the fastest-growing online retailers, Gilt Groupe. "If you do a lot of little things, you'll find the big things that scale," she said on a panel called "The New Consumer Conversation" this morning at Fortune Brainstorm Tech.

While experimentation tends to be a best practice of youthful companies like Gilt and Facebook (whose VP of Partnerships and Platform Marketing, Dan Rose, joined Lyne on stage), the role model company turned out to be a middle-aged Fortune 500 stalwart: Starbucks (SBUX).

Lyne recalled getting a phone call out of the blue one day. "Hi, I'm Howard Schultz. I have a company called Starbucks," the caller said.

"I know," replied Lyne, amused and very intrigued.

Schultz asked Lyne how he might use Gilt's flash-sale platform to treat Starbucks' best customers to a special deal . What they ended up doing together was a three-day sale of Galápagos San Cristóbal coffee, a rare blend from the Galápagos Islands. With that offer of the micro-lot brew, Gilt pitched a tour of the Galapagos Islands and other merchandise. The email offer went to 450,000 Starbucks cardholders; 45% of the recipients opened the email, Lyne said, and the coffee sold out the first day.

That was a big hit, as was Volkswagen's (VLKAF) decision last December to sell three new Jettas for a socking $5,995--about a $10,000 discount--on Gilt. The flash sale drew 725,000 interested customers--and 56,000 agreed to put their names on a wait list for more sweet offers. Volkswagon sent those people $500 coupons--and turned some of them into full-price buyers.

Whether you're a startup or a Fortune 500 giant, engagement requires a personal touch from the top, noted Stephen Gillett, Starbucks' chief information officer and EVP of its digital venture arm. His boss, Schultz, for instance, frequently posts on Starbucks' Facebook page, reaching out to the brand's 24 million fans.

Starbucks, in fact, was a pioneer in social engagement, via MyStarbucks.com. Asking customers what they want, via that site, has generated 156 ideas that have seen the light of day, Gillett said. Coffee-flavored ice cubes didn't make the cut. But mobile payments and free wifi made it into Starbucks stores quickly because engaged customers asked.