The pace has picked up on people registering NNNNN.com, which mean five number .com's. For example, 90210.com or 53762.com
There are only 100,000 possible combinations, and at the time of this writing probably less than 7,800 are available to register. Yesterday, there were over 10,000 available. Domain forums have a lot of discussion on NNNNN.com getting sold out, so that is probably increasing the pace of registrations.
Three number domains and four number domains (NNN.com and NNNN.com) sold out long ago, so the next shortest numeric domain that is still possible to register are the five number .com's.
100,000 might seem like a high number, but when you consider there are 6 billion people on the planet, it isn't so high. In addition, one company allegedly owns over half of these types of domains for use as US zip codes.
Some people speculate that it is an 'artificial' buyout, driven by domain investors only. But over the past few years, we have seen short three or four number/letter .com domains increase in price substantially. The thinking is the same thing will happen with NNNNN.com. Even if they attain a re-sale price of 20 to 30 dollars in the next year, that is still a 100% to 200% increase on an investment, which isn't bad at all. Every year domains have to be renewed if you want to keep them, however, which typically costs 7 to 10 dollars.