When young maize crops began failing in parts of Kenya earlier this month, the bad news—as well as information about where farmers could get seeds for other crops—spread on many Internet sites, including Facebook, which has 38 million users in Africa.

But it was a mobile platform called iCow—which allows 11,000 farmers and other members to receive livestock-management and other agricultural information—that helped cover the crucial "last mile" to older farmers. When a message from iCow passed along a tip already posted on Facebook about disease-free seeds available from the Kenya Agri Research Institution, that institution was, within two hours, besieged with hundreds of calls.

"Facebook has got the younger farmers on it, and iCow has the older farmers on it. We can bridge that gap to the older farmers who don't have access to Facebook and don't use the Internet," said Su Kahumbu, the founder of iCow. 

The episode is a reminder of the limits of Facebook, and of the role that small, mobile platforms and mobile-focused social networks can play, especially in the mobile-centric and culturally and ethnically nuanced African market.

Facebook did not start out as a mobile platform and is still playing catch-up on mobile applications—witness the fact that it felt compelled to spend $1 billion on the mobile-only Instagram photo-sharing app. And recent moves in Ghana and South Africa show that Facebook will continue to get a run for its money on that continent.

At the end of 2011, Africa had a population of just over a billion people, and 140 million Internet users. Despite the upstarts, Facebook is still growing fast in most African countries and is the dominant network in most of them. But not in South Africa, where Mxit has 10 million active users, more than double Facebook's number.