The question is whether they can monetize despite lack of purchasing intent (as compared to Google, Amazon, Yelp, etc). However from some anecdotal evidence[0], it appears that growing revenue may not be as difficult for Pinterest as it is for Facebook or Twitter.
I'm pretty bullish on Pinterest, and a $3.8B valuation doesn't seem out of line compared to Twitter ($20B) and Facebook (~$80B pre-IPO), Instagram ($500M).