While Waaqism was not certainly completely monotheistic, you can find similarities with East African religions and Islamic monotheism due to our afroasiatic heritage. I think even some nilo saharan pastoral religions are less idolatry driven than other African religions. In other words, a lot of beliefs about Waaq correlated with Islam. Hunter schlee, the German who researched the Rendille, remarked how a few of their beliefs were similar to Islamic and Middle East beliefs about God.Check out Sada Mire's "Divine Fertility", we don't have any statistics or even estimates in terms of adherents but there's archaeological evidence of Jewish/Christian practice in Somaliland at the very least. The Arabian peninsula had a long maritime history with the Somali coast going back to antiquity, it was inevitable that Islam would spread due to trade relations and the missionary efforts of Sufi shaykhs. People like to talk about the idea of pre-Islamic Waaq beliefs being monotheistic, hence Somalis having an affinity for Tawhid when they encountered it -- I don't buy this, because that tradition wasn't monotheistic (it was henotheistic) and engaged in ancestor worship. An important notion that was latched on to was engaging in Ziyara and receiving Barakah from the tombs of these shaykhs, who went on to become the 'ancestors' of the major clans.