In the short term, no one will likely bother to copy the code and open a competing (or even related) site. So business-wise, opening the code doesn't have any short-term impact.

In the longer term, if they were to become hugely successful, then many people would love to copy them and having access to the code would make that very tempting. But "hugely successful" is a big if. By then, they probably would have enough of a community that copycats wouldn't hurt them too much.

We have been toying with similar ideas for our startup, especially since its concept lands itself very well to community-based development. No decision yet.