The bottom navigation bar inside the Facebook app on my iPhone 7 Plus has five icons. There's one for the News Feed, followed by videos, Facebook Marketplace, notifications, and lastly, a button to see more options, such as my profile.

If I were using an iPad in Ukraine however, I might only see three icons: one for the News Feed, notifications, and one to see more options. If I were in Taiwan, Malaysia, or the Netherlands, I'd likely see a button for my friend's list.

These differences exist because it appears Facebook is aggressively testing its mobile navigation bar, or the series of tabs that run along the bottom of the app. There are over 60 distinct versions of the feature worldwide, according to a crowdsourced spreadsheet created last week by digital designer Luke Wroblewski.

Wroblewski's spreadsheet provides rare insight into how Facebook's designers manipulate its app in the hopes of changing user behavior. The social network has over two billion monthly users, and therefore plenty of guinea pigs on which to test subtle tweaks.

Facebook makes money by convincing people to spend as much time within its app viewing advertisements as possible. If a feature is clunky or hard to use, it could frustrate someone into closing the app and opening another instead.

In this sense, design is crucial to Facebook's (and any app's) success. It might seem excessive to test over 60 subtly differentiated navigation bars, but the social network is motivated to find the perfect iteration that will please people the most. It also needs to consider the fact that its users live all over the world.

For example, in different regions, Facebook is likely concerned with popularizing certain aspects of its platform more than others. If Facebook Marketplace is disproportionately popular in one country, it makes sense it would want to promote its use in that country.

There are also other regional concerns that Facebook takes into consideration. In some countries where high-speed internet connections are more rare, a simpler version of Facebook is often used, called Facebook Lite. The navigation bar in Facebook Lite doesn't include a button to view videos, because they gobble up mobile data.

As several people who contributed to the spreadsheet also pointed out, Facebook may be changing its mobile app based on how an individual user behaves. "I save a lot of stuff for later… So I have a 'saved for later' icon," one person wrote. I asked Facebook whether it tweaks the navigation bar based on a specific user's actions, but it wouldn't tell me.

"We are always exploring ways to improve the Facebook experience, and are currently testing different navigation options," a Facebook spokesperson told me in a statement.

Facebook regularly tests changes to its platform. In September, Motherboard spotted that it was trying out a Tinder-like feature inside its Messenger app, for example, and Facebook is constantly tweaking its News Feed algorithm. The social network famously allowed researchers in 2014 to manipulate the News Feed to try and determine how it affected people's emotions.

As Facebook continues to take into consideration criticism and grow its business, it needs to tweak its design to reflect those changes. This won't be the last time Facebook's experiments become apparent, but it is an important reminder of how the social giant is constantly thinking about how to influence our behavior.

But as one Facebook designer pointed out on Twitter, it's probably not worth reading too much into the navigation bar alone. "lol at everyone acting like the FB nav bar is the next Da Vinci Code or something," he wrote.

If you spot another version of the navigation bar Wroblewski has yet to document on his spreadsheet, you can send a screenshot to him by replying to his tweet here.