SAN FRANCISCO » If you’ve ever sent a virtual gift to a friend on Facebook — perhaps an image of a tasty-looking cocktail — you may have wished that icy-looking, pixelated glass was the real thing.

Now it can be. A service called Bartab is making it possible for friends to use Facebook and a cell phone to send each other vouchers for actual drinks that can be collected at local watering holes. The drinks are cheap, too, costing the sender and recipient $1 apiece.

Bartab is the first service from San Francisco-based startup Webtab that enables what CEO Steve Johnson calls "social transactions." Basically, Webtab wants to help people begin transactions on the Web, such as buying a drink or a meal for a buddy, and finish them up in the real world.

Bartab is one of a growing number of services looking to make money by combining our online and real-world social lives. It joins group-buying sites such as Groupon, which advertises local deals that only work if a large number of shoppers commit to buy, and location check-in services such as Foursquare, which give shops, restaurants, bars and other businesses a way to find and market to loyal customers.

At the moment, Bartab is available only in the San Francisco and Los Angeles areas and in New York, but Johnson is planning to add 17 markets in the next few months, including Seattle, Atlanta and Houston.

Here’s how it works: After signing up for the service, you pick a bar and a drink that bar offers through Bartab, such as the $1 margarita at Tres Agaves in San Francisco. Then you decide whom to send it to. Since Bartab uses Facebook Connect to identify users, it lets you search through a list of all your friends from the social-networking site.

The drink purchase shows up on that friend’s Facebook profile, and you can also alert them via text message. The recipient, who must sign up for Bartab before imbibing, can claim the beverage at the bar within three months.

If your friend uses Bartab on a computer, she instructs the service to send her phone a text message as she’s headed out to the bar. When she’s there and ready to drink, she replies to that text and gets back a time-stamped coupon for the $1 drink that is good for five minutes.

Bartab’s iPhone app makes it simpler. At the bar the recipient clicks to claim the drink, and a screen appears with a timer and instructions.

Consumers seem to be catching on. More than 30,000 people have signed up for Bartab so far, with users sending buddies cocktails, shots and beer at bars such as Nola in Palo Alto, Calif., and Flight 151 in New York.

Greg Gilman, 36, signed up for Bartab in late August and sent his girlfriend a drink at a bar called Rush Street near their Culver City, Calif., home.

Gilman, who runs a health care technology startup, also quickly invited eight of his closest friends to join Bartab, hoping they’ll be interested in it, too.

"I just thought it was a brilliant idea," he said.

Johnson formulated the idea for Webtab while working at an Internet-phone technology company called Lightscape, after being introduced to Facebook Director and former PayPal CEO Peter Thiel by some friends.

"I thought, everyone is doing these little one-off applications that do virtual things. Why don’t we put together an application that makes real transactions happen on Facebook?" he said.

Webtab sprung from this query, though Webtab did not beget Bartab. The service was originally built as a Facebook application by students at the University of Arizona, serving just people in Phoenix and Scottsdale, and received a $25,000 grant from Facebook’s fbFund startup incubator in 2008.

Webtab purchased Bartab late last year, using an undisclosed portion of the $2 million in funding it has raised. Webtab reworked Bartab to serve lots of people in different cities and relaunched it in May.