Not sure it applies only to multiplayer. Let's take Popcap's PlantsVsZombies which was single player paid game. PlantsVsZombies2 might be some kinda' free2Play model. A free2play version is actually easy to imagine. There'll be an item shop where players could purchase bonus traps, not included in the default trap set for example. Mass Effect free to play version? Imagine you can play whole game without half the characters, no bonus areas, fewer weapons and so on. That or some kinda` ad based model, where main characters drink galactic coke, wear Adidas jumpsuits and find old poster of Lady Gaga in abandoned ancient city.
Here is what PopCap had to say about the layoffs and free to play:
"Free-to-play, social and mobile games have exploded in popularity. That happened fast. Surprisingly so. The change in consumer tastes requires us to reorganize our business and invest in new types of games on new platforms. It’s a completely different world from when we started. [...] We’ve been able to invest in creative new games like Peggle and Plants vs. Zombies because we had a high profit business. That business is challenged, and if we don’t adapt, we won’t be able to invest in new IP."
...also about those layoffs someone mentioned earlier:
"Vechey also points out that even after the cutbacks, PopCap will end the year with roughly the same number of employees it started the year with."