When Killzone 2 was going to release, it was hyped as a Halo killer by many outlets, but whilst it didn't live up to those "unrealistic expectations," it did live up to the hype in terms of it's graphics and gameplay (although this is debatable).
It's nice to know then, that this has been recognized by gamers, as the game has sold 2 million copies according to VGChartz (yes it's slightly unreliable, but it's better than nothing right?)
The game has sold most in Europe during the last 33 weeks since release, but recently the game has seen a slight resurgence in sales in Europe. It was previously selling around 7,000 units weekly, but for the last four weeks it saw a steady increase to approximately 20,000 units this week. The same boost was not seen in America or Japan.
It is very likely that the game's sales were boosted by the newly relaunched PlayStation 3.