Slowing down the rate of progression creates reasons for players to spend real money. But it is important to keep in mind that player portray their progression based on the progression of their playing friend, which is why virality is super important for monetization.

Ok, but back to the topic. So you have players playing the game with their friends. Sooner rather than later one of the friends will start falling behind. 

Whatever the reason, some players will want to progress faster than the game allows and will happily spend money to do so - especially if it will allow to catch up and surpass their friends. This desire to progress faster is the number one reason players spend money in a social game

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New player is primed to spend when the progression hinders significantly 

Player is "primed to spend" when they have friends in the game and feel a social obligation to keep up. At this point the game's progression curve needs to flatten out - takes longer to get to the next reward so that spending money (or going viral) to save time and progress faster becomes more and more attractive.

Creating Prime to Spend moment

So in theory prime to spend moments are born when player's progress slows down and player reaches for the pocket in order to catch up or pass playing friends. In practice designers can create these moments via cleaver use of two kind of quests - trunks and branches.

Here's how a trunk a branches could look like in a game

The trunk

of the quest tree is made of quest, which carry on the game story. These quest are mostly made of grinding and periodically blocked by gatekeeper quest.

Example of a gatekeeper:

In most of Zynga's games player encounters the gatekeeper quest after some 20 minutes of active gameplay. At this point player is usually unable to finish the second important building without purchasing premium currency or without asking for the needed building items from friends.

The branches

of the quest tree is made of quest, which don’t carry the game story and which can be completed without the help from friends or turning to premium currency. Nevertheless these quests are very important due to 

following reasons:

1. They prolong the game. More quests usually leads to more time spent playing. More time spent playing converts to better retention and better retention is more people playing the game.
2. They give a sense of progress. Completing few short quests while at the same time progressing in the gatekeeper quest is much more fun that just grinding with the same quest for a week or some.
3. They help to prepare players for following quests. Branch quest may be all about planting tens of crops but after this is done player has more resources and isn’t intimidated by a gatekeeper quest, which demands lots of resources.