Comment by Freak_NL

Network effect. Pinterest has claimed its niche (arts and crafts, wedding inspiration (the size of this niche never ceases to amaze me), home decoration, pictures of smoothies in mason jars (but never with a matching lid oddly enough), clothing, accessories, etc.), and has its share of loyal users. Any competitor would need to be at least twice as good to attract enough users for the network to shift to them.