In September 2013 the penetration rate was estimated at 70% over a population estimate of 168.8 million.
21.59 million subscribers at the end of June 2013.
Nigeria has 120 million mobile subscribers for 69% mobile penetration rate per capita (according to latest stats by Ericsson) and Nigerian smartphone population is 27 million (Ericsson) so about 23% of Nigerians who have a mobile account, are using a smartphone. Thus 77% of Nigerian mobile subscribers still use a 'dumbphone' or featurephone. Or if targeting featurephone technical abilities (SMS, voice, USSD, MMS, WAP) you reach more than 4 times larger audience than doing all smartphone platforms - combined (which in Africa includes large portion of Symbian still).
About 80% of Nigerian handsets are on 2G (source: Etisalat) and thus only 20% on 3G or faster, again the very basic services such as SMS, voice, WAP, USSD are part of the normal serices mix in the Nigerian (and other African) market(s). About 8.5% of the total GDP of Nigeria comes from the telecoms sector and before you say 'fixed' - landline fixed connections are less than 1% of total Nigerian telecoms. So yes, well over 8% of the total Nigerian economy is driven by mobile.