San Francisco ? Facebook and Twitter have prodded people to broadcast just about anything. Now a new site wants to ask people to automatically reveal things they buy.

Blippy, backed by a Twitter co-founder, asks people to share their spending habits. If you register a credit card with the site, every transaction on the card would be displayed to your friends on Blippy.

While there are plenty of Web sites on what people are purchasing, the site’s founders think it’s a new way to learn about deals and products. Knowing your spending habits are transmitted might make you think twice before spending. Charities could use Blippy to show that they use donations responsibly, too.

Co-founded by entrepreneur Philip Kaplan, the joker behind a profane Web site that mocked failed startups during the dot-com bust, Blippy encourages you to connect credit cards and accounts at e-commerce sites to your site profile. Then, whenever you buy in person or on the Web, the purchase is posted for your friends to see and comment on. They’d see something like “Joe1234 spent $2.98 at iTunes.”

Some purchases are more descriptive than others. If you buy an iPhone game Blippy can show its name, not just how much you paid. But for now, if you spend $250 at a grocery store with a linked credit card, Blippy would indicate the total amount rather than every item.

Blippy doesn’t store its users’ credit card numbers. You give the site a username and password to access your credit card account online. Other sites, such as the popular personal finance site Mint.com, have a similar setup.

For those wary of baring all, Blippy lets you hide individual purchases from your activity stream or make it so only approved friends can see your transactions.