Pinterest is a compelling story. As mentioned in the article, the main source of their monotonic growth early on was that their users loved the product, and personally recommended it to their friends. Seeing a product grow all by itself with little other advertising or marketing is always a good sign.

I actually use Pinterest myself to keep track of cool gadgets, books, and since I am in academia, papers, research, algorithms and ideas.

I've though about Pinterest's success for a while, and I think its special sauce has to do with permanence, where as most other sites are currently focused around recency. Posting an item to Pinterest feels additive: I am collecting and curating good content, and the entirety of my work is displayed for all to see in very few clicks. On the other extreme is Twitter/Facebook/Google+, where I've even stopped posting good content unless it falls in specific time on a weekday, or I know that it will sink into oblivion, never to be seen again. The time of a post can easily be the difference between 0 and 50+ retweets. (I tested this.)

The other major aspect of its success, I believe, is that Pinterest is very visual. This simple truth is obvious, but rarely executed on: People love pictures, and they don't like reading too much.

So Pinterest is doing many small things right, but more importantly it's filling a gap in people's desire to express themselves. Luckily, the team realized this and stubbornly believed it.