In Mobi, one of the world’s largest independent mobile ad network, has released its May 2011 Mobile Insights Report for Africa, providing insights on mobile advertising trends on the continent. Report shows that InMobi now serves 4.2 billion impressions a month in Africa, up to 37.9% from 3.0 billion in February 2011.

Strong growth in Nigeria continues with mobile ad impressions doubling in just 90 days driven by the 456 million growth in impressions in standard Internet-enabled phone impressions. Smartphone impressions increased by127%, but remain relatively small with only 11% of the total market share.

Nokia OS and Symbian OS collectively command 78% share with no significant threat yet from Android, RIM OS, or Apple iOS.

While the data indicates that standard Internet-enabled phones and smartphones impressions grew at a consistent pace, most regions across the globe shows smartphones growth outpacing advanced phones, highlighting the relatively slow pickup in the region.

Vice President and Managing Director, InMobi Africa, Isis Nyong’o, confirms that “Mobile media continues to consume the mindset of consumers across the continent. As mobile ad impressions continue to grow rapidly. It’s evidence that more consumers are using the mobile web and current users are highly engaged.”

The report finds that Botswana, Ethiopia and Nigeria show the fastest mobile ad growth in mobile markets in Africa from February to May 2011. Nigeria and South Africa maintain the top two positions as the largest African mobile markets, and now represent 2 of every 5 mobile ads across the continent.