At the time of acquisition, I think the Instagram brand was more popular and recognized among users than Jet.com. Jet.com didn't become a phenomenon before Walmart acquired it. Instagram was already extremely popular when FB bought it.
If you peruse the HN comments from the time, many of them were convinced that this was a waste of money (though not the top comment, interestingly enough): https://news.ycombinator.com/item?id=3817840