Personally, if you're just planning on making a Facebook app, I hope you're doing it for fun, or to satisfy your own app desire, or with some eye towards a future opportunity. Making a Facebook app interface for an independent site makes a lot of sense, but I would not want to center my entire business model on a closed platform that has shown little evidence of revenue with the constant threat to have my functionality mirrored and obsoleted. So in that sense, I don't think the writer of a Facebook app has to worry about a cofounder, since said writer would (hopefully) not be expecting to found a company out of the venture. That would be like saying someone starting an open source project needs to wait for a cofounder (ignoring, of course, that whole "open" part).
I guess I'm trying to say that there are plenty of Facebook apps (free gifts, HTML walls) that are "viral" and easy to build, but how many of these have any chance at making money? I'm implying that any Facebook application that is useful enough to become a "founded company" will involve an external site of some sort, one that involves the same types of inherent complications that are present in any other startup.
What do you have in mind that can be built almost entirely in Facebook and expanded later on? I suppose if I think hard enough I can come up with some long shot ideas that may or may not work out (product-rating apps which could be expanded to include other social networks, for one), but even something like this is going to require some kind of heavy backend analysis to be especially useful. Keep in mind that we're also ignoring the (very real) possibility of Facebook sweeping in and mirroring any successful application or approach to generating revenue.
On another note, what makes Facebook any easier for experimentation than Apache? Writing scripts that print pages to a browser is as easy as writing to standard output.
Wait... which app is that income for? SocialMedia or Appsaholic or something else entirely? And they have no large infrastructure behind the scenes?
Going back to amichail's original comment, the implication was that making Facebook apps is really easy, and therefore a cofounder might be less necessary. The only point I was trying to make was that any Facebook app that is going to be really easy to make is unlikely to be a big revenue generator. My assumption was that any app that was going to be big enough and useful enough to generate good revenue was going to require a significant backend external website to crunch numbers and store relevant info.
So what's the point of what I'm saying? I was trying to counter the idea that building a revenue-generating Facebook app would be any easier than building an independent site. In both cases, you have to build the backend, anyways. So then we're merely talking about the difficulties of building the interface, and I was saying that it's not that hard to do with Apache and a script in just about any language: just print what you want the browser to see to standard output. Even if building a Facebook interface is easier, that is fraught with other complications (such as worrying about Facebook itself doing its own, official version of your app).
So in the end, I'm saying that building a successful Facebook app is not much different from building a successful website, so if you need a cofounder for a website, then you also need one for the app.