At first, Pinterest monetized its pages with SKIMLINKS. Users would post a picture link to a product that could be purchased, for instance, on Amazon.com using their own affiliate code so that they might receive a commission if a sale was made. Unbeknownst to them, Pinterest was wiping off these affiliate codes and replaced them with its own. This caused some outcry - largely becaues Pinterest did this secretly - and Pinterest dropped Skimlinks late in February 2012.
Now Pinterest is funded by venture capital to the tune of $38 million, with an eye on a future sale of the company or a steady income stream. Pinterest is expected to be monetized some time in the future.
This future may come soon. Just a few weeks after Skimlinks was suspended, on March 22, Business Insider announced that Tim Kendall had been hired by Pinterest. Tim Kendall's claim to fame is to have masterminded the monetization of Facebook pages.
There is money and advertisements in Pinterest's crystal ball.