An awful lot of people in this day and age. Just pick a popular conspiracy theory and punch it into Google. You'll get thousands of hits. Conspiracies are sexy. Who wants boring old science or 'most likely' explanations when you can have a much more entertaining explanation that involves cover-ups at the highest levels of government.
I guess part of the problem is that any of the stories could be true. Taking an example, let's suppose the 'face' on Mars really is a face and it was built by an intelligent civilization living on Mars. In order to pull this off, you've got to get a large number of people at JPL, NASA, etc in on the plan to retouch any new images. The face has to be doctored so that it doesn't look like a face anymore, along with any other evidence that might be photographed. You've now got dozens, if not hundreds of people in on the conspiracy, yet somehow they manage keep it a secret, at least until someone like Hoagland comes along and exposes it.
To me, this is just a little too much to believe. Three guys can't rob a liquor store without one of them bragging about the deed to his friends, yet we're to believe dozens of scientists, who supposedly dedicate their lives to the pursuit of knowledge, are able to keep quiet. I'm just not buying it.