So firstly, what exactly do you classify as an "exciting" company? This is very relative, because what I find exciting, you may not. It seems you are mainly thinking about tech companies?
MXIt is a pretty cool company in that regard then.What about Kalahari.net? The African version of Amazon...or at least they seem to try to be. I think if you actually really look into this (instead of posting in a forum hoping to uncover a giant of innovation that you somehow missed) you will find that there are plenty of exciting companies in South Africa, but that they are relatively small. Webafrica might be considered as a cool/funky/exciting company, though it too is very small.
South Africa is a very small market. Most people here live in poverty. Of all the kids starting Grade 1 this year, maybe 30% will actually make it to matric. Less will pass. On top of this, we have all these stupid regulations on how many people of what color need to occupy what positions, and having worked abroad for multi-national firms, I can tell you that in more than one instance, this was the primary area of concern for them and reason not to invest. On top of all this, there seems to be less capital available in our country for startups, and not enough support for this from the government. These 'exciting' companies, like Google, Amazon etc. have offices here sure, but they are small offices, for the above reasons and/or for some of the other reasons mentioned so far. Further more, SA seems to have a much more conservative and traditional approach to business and change. We're much more formal - which is probably a contributing factor to finance being one of the dominating industries here. But that (finance) is probably not exicting to you!
These are not excuses. These are simple, basic, facts about South Africa. We're on the African continent. We have limited skills, a limited market, limited capital, and often a limited mentality to boot. For all of the above reasons, if you want to do something 'exciting'....or to stick with your examples...revolutionarily tech....you need to probably in the first instance make sure it has global appeal so you can hit a global market. Lets say you come up with Social Network 3.0 or whatever......why operate from SA? The venture capitalists for the most part probably wont get your idea, probably cant offer that much capital anyway, you wont find the right skills or be able to get them into the country very easily if you do.
This is not to say that innovation is dead. It's not. It's everywhere. But its a challenging environment. You can be very successful in SA though not always in the realms that you regard as exicting. Read about yola.com and Vinny Lingham for an interesting journey.