On an unrelated note, how is the form at the bottom supposed to update you when UberPool is in your area? Area codes are going to be inaccurate for anybody who's moved since they got their cell phone number.
Uber have a lot of advantages. They have a userbase of passengers and drivers that can be brought over to new services. They have the media's attention, including mainstream media. They have the cash to run at a loss while building up. They have presence in a lot of cities. They also have the competence to do this well.
Imagine that you build a "person search." You enter a name. It brings up a page with pictures, facts and other information collected around the web. It's nifty. That's interesting. But, if Google put a little "people" link between images and news in their search page and person search will have hundreds of thousands of users this week.
The news here is "uber launch carpooling service," not "carpooling service now exists." Rightly so, IMO. If this works well, there could be tens of thousands of user in a couple of months.
But you're right, the key to this is the implementation of the idea over the idea itself. Uber (probably) will generate a larger network of car poolers, which is necessary for this type of service to work.
For a general near-real-time hailing system, limiting it to a smaller area means that even if your best match would add a relatively high distance penalty (say 50%), in absolute terms that's still tolerable (a few extra minutes). So that increases the probability of finding a match right now where the economics work out.