Ordinarily, I'd flag this as an opinion based question, but underneath is a lesson on project scope, one that is very timely as we approach the class deadline at the end of the year. I've never written a Facebook app, so I can't speak directly to that, but I can offer some advice.
You have the idea for what to do and you know your skill level. Now, spend a day or two writing a specification for the project. What needs to be done? What are the major milestones? What are the specific significant parts of the project that have to be written? What are all of the inputs? What are the expected outputs? What does the app do? Do you have access to all the resources, toolkits and SDKs needed? Can you outline all of that, or is it still just a big, vague concept? Is the project still a 'simple app' or far more complicated than you thought?
IF you can write all that out in some detail in a day or two, then there's a good chance you can do it. If you can estimate how long it will take to create each part of the project, then you have an even better idea.
There's another way to look at it too. You're done with everything except the project. The deadline is the end of the year. You could roll the dice and finish the project, or if not, retake the class next year and redo everyhting. Most of the psets should be the same and you'll have plenty of time to finish the project. ;-)
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