Players can use those coins to buy more crops, livestock or other items for their farms, such as picket fences and gazebos.
Crops grow or ripen over different periods of time -- hours to days in the virtual world.
The game's players have virtual neighbors, other Facebook users who also play. Neighbors can help each fertilize crops and get rid of rodents, and they can also send each other gifts.
FarmVille recently added features where users can use real dollars to purchase virtual coins and FarmVille money.
In fact, 15 percent of the total number of FarmVille players -- or 12 million people -- actually spend real money on the game.
Five percent of players spend at least $20 a month to get "coins" to allow them to advance in the game faster than competitors who play for free.
The play-to-pay option and advertising revenue are expected to make Zynga $450 million this year alone.
FarmVille outsiders don't understand the fuss of building a virtual farm or gifting virtual farm gifts with others.
Young said the social component of the game was part of the allure.
"It connects many people that might not normally be connected. They have a common goal, they do like to show off their farms."
Here are some of Dr. Young's tips for what you can do if you find yourself playing too much of any online video game:
Set an egg timer next to your computer and monitor how much time you spend playing the game.
Keep a journal of all the times you are on, then review the journal periodically. You'll probably find that you visit the game more than you realize.
Limit your computers to one in the family room so everyone has to share.
List the consequences of your absorption with the game, such as forgetting to pick up dinner or being late for work.
ABC News' Toby Phillips contributed to this story.