I guess you are not really aware how ratings work. The millions of viewers do not count and I wasn't talking about it. What matters is the adult 18-49 demo rating. How many viewers between ages 18 and 49 watch a show is what interests advertisers. Person of Interest gets a demo rating around 3.0 every week. While Unforgettable was getting around 2.0 which by CBS standards is considered low. But, it was a mistake they initially cancelled it because Vegas this season is getting a lesser rating than Unforgettable. They should have just moved it to Fridays where they premiered Made in Jersey and canceled it after 2 or 3 episodes this season. Unforgettable S02 will now air in the summer of 2013.
Any way, Person of Interest is the second highest rated drama on CBS. NCIS is the only drama that gets a better rating than PoI on CBS. There's no way CBS will cancel it. Trust me, it's a lock for a third season renewal. Consider this as an official confirmation. ;-)
P.S. A brief explanation about ratings:
As of August 2012, there are 126.54 million adults (18-49) in USA who live in a household with a
TV.
A 1.0 demo rating will equal to 1.265 million adults.
So, a 3.0 demo rating will approx. equal to 3.8 million.
In a typical week, PoI gets 15 million viewers, but it gets a demo rating of 3.0. In other words only 3.8 million of those 15 million viewers are aged 18-49. This is the audience that advertisers target and a show lives or dies with this rating.