Like they have tons of users who religiously use their product, and it's actually coupled with intent to buy, which makes monetizing easy.
However, you can't change the fact, that pinterest is riding on a "trendy" wave right now. There is not a whole lot that keeps you threre. Just in two years, there can be another spot where all the women in world woo about their lives.
It's an inherent social problem which accounts for the so far pretty short lives of social companies. You gotta have to be able to tie/commit your users much like Facebook does (owning your diginal private life).