Which gaming product innovation first popularized the 'free-to-play with microtransactions' model that now dominates the industry?

In the rapidly evolving world of gaming, business models have transformed dramatically over the past decade. One particular innovation revolutionized how games generate revenue and engage players through virtual economies. This innovation changed the landscape of gaming from one-time purchases to ongoing engagement and microtransactions, creating entirely new product strategies. Test your knowledge about this pivotal shift in gaming business models!

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Social network games like FarmVille (2009)
Subscription-based MMORPGs like World of Warcraft (2004)
Mobile game app stores like Apple App Store (2008)
Early console downloadable content (DLC) for Xbox 360 games (2005)