I'll offer some brief bullet-point answers and then close with a slightly longer thematic explanation as to why I think Pinterest is gaining traction. Briefly:

• It emphasizes pictures over text.
• The site has a fun and light brand, logo, look, and feel.
• The site has kind of created an inverted "wish list," something Amazon tried to do but didn't really succeed at.
• It has initially appealed to mostly women (who of course have better taste than men!), who have in turn pinned and re-pinned more interesting content.
• It is relatively easy for new users to sign up and then crawl the net to "pin" images they like and "re-pin" images from others they follow. Furthermore, the act of creating "boards" and pinning things to them appeals to users' organizational sensibilities, like filing things in folders, and so on.
• It has generated terrific "word of mouth," especially among women and Silicon Valley investors :)

The main reason I believe that Pinterest is seeing a rise in DAUs and MAUs is because it has tapped into a potential shift in consumer behavior and purchasing behavior.

(Before I lay this out, I should preface the remarks by saying that it will take a long time to shift this behavior, even if it's possible, and during this time, intent-based search will still be powerful and lucrative.)

The shift that is happening is from search to discovery. Everyone talks about this shift, but what does it mean?

As we make a decision online to purchase something, we are trained to go to Google (or Amazon), type in the keyword, and sort through results to make a transaction. In return for that sorting, Google makes a lot of money, but in order for it to work, we individuals have to signal our intent: "Black Nike running shoes."

But, how did I decide to even want these running shoes?

While Google makes money at the bottom of this decision funnel, the top of the funnel is where "discovery" happens. It's much wider at the top of the funnel, and hard to pin down where the thoughts originate. A site like Pinterest could help bring some of that discovery online. Say that I'm in the market for new shoes, but instead of researching them myself, I elect to browse the boards of some friends on Pinterest who I already know are dedicated runners? I could find a pair of sneakers on a friend's Pinterest board and have reasonable confidence that this pair would suit me, too. In this manner, I may elect to buy the shoes right after seeing my friend's board on Pinterest and get to a transaction much quicker. If Pinterest can bypass intent-based search on just even a small fraction of online transactions, it will be a huge success.

This is the promise of Pinterest, and this is why people are signing up like crazy (because they want to discover online) and why, in turn, investors have been trying to invest in this trend. The trick, however, is that if users aren't shown dynamic retail pricing for items directly on Pinterest, users may still go back to Google to find and locate the best price and inventory for a purchase.