Needless to say, this has been an interesting week for Facebook, Pinterest, and marketers.

After winning $100 million in a new round of financing, Pinterest, the scrapbooking Web site, became the latest start-up to join the billion-dollar club since it is now valued at $1.5 billion, reported the Wall Street Journal.

Marketers are starting to pay attention to Pinterest since not only are users flocking to it (ComScore has said Pinterest is one of the fastest-growing stand-alone sites it has ever tracked), it is also a boon for brands. According to data from Shopify, an e-commerce platform for building online stores, referral traffic to retail sites from Pinterest is equal to traffic coming from Twitter and Pinterest users spend twice as much on an average online order than Facebook users do.

Although Pinterest does not accept advertising yet, the latest round of funding further cements the chances that this start-up is here to stay. Huge brands like Whole Foods and the Gap have already created their own “boards” and it would not be a surprise if more companies join in as well.

As for Facebook, while investors keep a close watch on their newly minted shares of Facebook stock, marketers are waiting to see what the social network’s next moves will be in advertising. In addition to hosting its first ever marketing summit, Facebook has been experimenting with potential revenue-generating offerings, such as “Sponsored Stories,” ads on the log-out page, coupons for local stores, a paid app store, and more.

General Motors made some noise earlier this week when it yanked its entire $10 million ad budget off Facebook, prompting more discussions about the difficulty of justifying paid advertising on social networks. Regardless, experts predict that Facebook will soon be rolling out even more ad formats and targeting capabilities. “It’s a big win for marketers, giving far greater reach and hyper-targeting abilities,” Dave Kerpen, CEO of Likeable, a social media agency, tells Mashable. “So get ready to hone in on your perfectly tailored audience.”