The next couple of months will make for an interesting case study on how tech startups defend (or not) themselves in the face of a massive competitor checking in its tanks on their front lawn.

Foursquare's PR team (which was probably two people a week ago) will, I suspect, be getting schooling and guidance by an array of the best PR brains their VC's address books can offer. They've already scooped a massive advert in Times Square and will now hammer the media about growing user counts, why they deserve to exist and why Facebook doesn't get it.

Used to be the case that Microsoft was the only one to defend against, but that's when the tech industry's metabolic rate was much slower. I suspect we'll know within a couple of months if Foursquare has a future.