When you have a game enjoyed by 75 million people, you have to make changes very carefully. That’s why Zynga has been slowly rolling out a new feature in its FarmVille game, which is the most popular app on Facebook.

The new feature is a FarmVille Game Bar, pictured above, which resides in your internet browser. This feature looks like it isn’t universally available; I got an invitation to install it today.

The bar is a constant reminder when you are surfing the web that you could be playing FarmVille instead. It has a “Play Now” button, a “bonus” button, and a timer for when you can harvest your crops. The latter is quite useful because if you don’t harvest your crops on time, they will wither and die, and you will have wasted your precious FarmVille cash. The game bar also has a Zynga button that I can use to install new Zynga games.

Whatever this game bar’s purpose, it seems pretty important as Zynga offered me 25 dollars in FarmVille cash to install it. Normally, I would have to spend real money to get that cash so I can buy virtual goods such as tractors in the game.

This addition is an attempt to get users to come back to the game on a more frequent basis. Zynga knows that if players do that, they are more likely to stay engaged with a game. And if they are engaged, they are more likely to become spenders of real cash, buying virtual goods. And the more free users converted into spenders, the more profits for Zynga.

When Zynga makes changes to its games, it does so scientifically. Brian Reynolds, chief game designer at Zynga, said his company routinely tests new features in the Canadian market, which is maybe 5 percent of the size of the U.S. market. The company views the game as a service that has to be constantly improved. Certainly, Zynga has to do something to keep this cash cow going. The game hit its peak a while ago at around 82 million monthly active users and since then has hit a slump, according to AppData. Zynga has to keep trying new things, particularly since Facebook has curbed the use of viral communications to spread the word about apps.

By the way, I feel a little bamboozled now. I installed the FarmVille game bar and my 25 dollars in FarmVille cash hasn’t arrived yet. What gives? [Update: it showed up in my gift section, where I wasn’t looking for it].