Ad revenue makes the site go round. Fanpop would shut down if there weren't enough users to keep the site going through advertising, especially now when the site seems to be doing well with advertising. Assuming that the site is profitable now (who knows?), if that money went away, the site would, too. The staff has to put food on the table, after all.
It's disturbing to consider, but the reverse may also be true, though: if the site starts making a LOT of money (presumably from millions of users), then the chances that the company will be purchased by a larger company increase. From an user perspective, that's all fine and good...unless the purchasing company decides to change how Fanpop operates, or rebrand the site to particular clubs/topics. Say (for a ridiculous hypothetical example) the Children's Television Workshop buys Fanpop. It's possible that it would do so as part of a plan to expand into social networking, but more likely it would be that the CTW would take the Fanpop technology and adapt it to operate clubs related to CTW properties, such as Sesame Street, Dragon Tales, etc. Rather than maintaining two separate sites, the Fanpop technology could be wrapped into the purchasing company's site, which in this example would be
link. One could make an argument that other social networking sites might be less likely to discard Fanpop, but at the same time, how likely would it be that they would operate two sites which might be seen to be in conflict with one another?