SEARCH engine Google and social media Facebook are at the forefront of a scramble to win over new African internet users, offering freebies they say give a leg-up to the poor but which critics argue is a plan to lock in customers on a continent of 1billion people.

Africa's internet penetration will reach 50% by 2025 and there are expected to be 360million smartphones on the continent by then, roughly double the number in the US currently, McKinsey Consultants data shows.

Africa had 16% internet penetration and 67million smartphones in 2013. This growth is attracting interest from internet companies such as Google, Facebook and Wikipedia, which are striking deals with service providers such as Vodacom, MTN, Bharti Airtel and Safaricom to offer users free, or "zero-rated" access to their sites and services.

Facebook, through its internet.org programme, offers a stripped-down version of its social network and some other sites for free in what it says is an exercise to "connect the two-thirds of the world that doesn't have internet access".

Google, in partnership with Kenyan mobile phone firm Safaricom, is rolling out its "free zone" in Kenya, where e-mail and the internet are available with no data charges, providing users stay within Google apps.

Google has said its "free zone" is aimed at a billion people without the internet in the developing world.

France's Orange is offering free access to a pared-down version of Wikipedia in some African countries, while South Africa's Cell-C gives its customers free use of WhatsApp, a messaging service owned by Facebook.

Critics, however, say big service providers and internet companies are luring African users into using their services, giving them opportunities for greater advertising revenue.

"It's like a drug pusher giving you a small amount and saying: 'If you want more, you have to come and buy it'," Africa internet access specialist Mike Jensen said.

Giving Africans free access to some internet sites may also stunt innovation and limit the opportunities for African entrepreneurs, making online technology another industry on the continent dominated by big foreign companies.

"You are giving people the idea that they are connected to this free open world of the internet but actually they are being locked up in a corporate digital prison," Niels ten Oever, head of digital at rights group Article 19, said.

"Where will the African Mark Zuckerberg come from when they have no chance to compete?"

There are also concerns that regulators in Africa lack the capacity to track how telecoms companies allocate bandwidth.

The 24 sub-Saharan African countries tracked by Internet monitoring site WebIndex have "evidence of discrimination" in the allocation of bandwidth and have "no effective law and regulations" on internet freedom.