I have a vague recollection in the back of my mind about somebody doing a survey a while ago - wasn't it Arthur Goldstuck/ITWorx? Could also have been ITWeb/Brainstorm, or the Computing SA people - I'm too lazy too check...
afaik there is an annual financial mail top 100 SA sites publication...as in recall news24 was on top as the most visited; i think the ranking had more to do with sites that save you time and money as opposed to most popular
but that is probably not so anymore because the stupid tossers have now put up a notice that SA's most frequently visited site is only available to mweb members - ffs
it being SA i am aware that keo.co.za which is a rugby blog gets in excess of 30 000 page impressions a day
if you are looking for "authoritative" stats probably the bets place is the online publisher's association (www.opa.org.za)