Why Facebook's 100M Africa milestone is huge

Facebook announced on Monday that it hit a new milestone: 100 million monthly active users in Africa—that's half of the 200 million people connected to the Internet across the continent.

Eighty percent of those Facebook users access the service on a mobile device.

This is the first time Facebook has announced user growth across a continent, and it comes as Facebook tweaks its product to become more accessible—and useful—to emerging markets' users.

Facebook
Jonathan Nackstrand | AFP | Getty Images

The milestone is important because it shows that Facebook is continuing to add users and provides a window into the social giant's strategy to scale in the fast-growing market sector.

Facebook's market share rose above $202 billion on Monday after the announcement, putting it on par with the world's biggest companies.

Earlier this year, Facebook engineers revamped its Android app to reduce its download size and decrease start times to make it more appealing to Internet users in developing countries, where devices limited memory was proving problematic.

Read More Facebook expands its video realm by news feed

This announcement comes just a month after Facebook's Internet.org announced an app for Zambia that offers free access to Facebook and a dozen other services, including apps with health and employment services, Google, and Wikipedia.

Just last week, Facebook announced bandwidth targeting, helping advertisers develop the best mobile experience based on the connection.

Read More What Alibaba learned from Facebook's IPO flop

Facebook is investing to create tailored products for advertisers to tap into this fast-growing market, saying it's working with partners—that include Unileveron tests of ad units based on how people connect with each other in different countries.

RBC's top pick: Facebook
RBC's top pick: Facebook   

In July, Facebook said it's testing an ad unit in India to tap into the phenomenon of people sending a message with a "missed call."

In India, when people see an ad on Facebook they can place a "missed call" by clicking the ad from their mobile device, in the return call receiving content, like music or cricket scores, along with a message from an advertiser.

Read MoreFB's new conference @Scale is all about geeks

Facebook is now working on a similarly targeted ad format for South Africa, where people will send an SMS text to signal that you should call. The company says that its South African version of the ads will include the functionality of the "missed call product" it's seen great success with in India, but with a different "call to action" to reflect local behavior.

Why the big push?

Advertising revenue in Africa dramatically lags the rest of the world. Facebook's worldwide average revenue per user was $2.24, but in the U.S. and Canada the figure is $6.44, and it's only 86 cents in the "Rest of the World" category that includes Africa.

—By CNBC's Julia Boorstin