Over the holidays, people have had more time to indulge themselves online. This has included a healthy exploration of Pinterest, the online scrapbooking service that has suddenly become the cat’s meow of the social networking world.

For those of you unfamiliar with Pinterest, it lets people “pin” photos and videos, and share them with other people. It has been a particular hit with women.

While still invite-only, it has captured the imagination of people as the number of users has nearly quadrupled to more than four million since August. As well, the company has raised $27-million in venture capital.

Where’s the Business Model?

The big question facing Pinterest is how it’s going to make money. Like Twitter, Pinterest is a company that surged out of the gate with no obvious business model, although it may have a plan that has yet to be publicized.

For those of us who believe startups should have some kind of business model out of the gate, companies such as Pinterest face an interesting challenge because it will have to introduce ways to make money that resonate with users without impacting growth.

Advertising & Sponsorship?

The most obvious money-making vehicle appears to be advertising and sponsorship given Pinterest is built around specific interests. This would let Pinterest leverage its content in a way that could be consumer and advertiser-friendly without ruining the experience.

E-commerce could also be an interesting opportunity given the growing interest in experiential shopping. Individuals or brands could create pinboards that marry interesting content with ways to purchase the products being highlighted.

Again, it would be a way for Pinterest to enhance the experience as long as it’s done properly. Pinterest could make money from charging a fee for companies to provide e-commerce, or it could take a cut of the action.

Pinterest could also explore adding premium services such as the ability for companies to get a customized URL or enhance their boards to promote their products and brands. This might not be as user-friendly but it could work if the customization subtle and natural.

While it could be challenging for Pinterest to introduce a business model that’s welcomed with open arms by users, the upside is having more than four million users gives it lots of options – a luxury any company would love to have.

For more on Pinterest, Mashable as a good tutorial the explains how the service works. VentureBeat has a post about how Pinterest has become one of the 10 most popular social media networks.