there's no blockchain
SAFE uses XOR networking which promises to be more scalable than blockchains - see also @rand_om's comments above which appeared as I was writing this!.
Safecoin as a cryptocurrency doesn’t exist as the system is not yet live. However, you can buy the MAID token at Poloniex and other places. Once the system goes live you will be able to exchange it for Safecoin.
I fail to find documentation to build "DApps"
There are demo examples here and the API is documented here. I’m not a developer but apparently the email app is a good place to get started.
there's a lot of platform out there waves,ethereum,lisk,counterparty.can someone please highlight why they stick to this project when there's no product yet and why should i build Dapps on this platform than others.
You are right in saying there are lots of platforms out there, but they all do slightly different things and in different ways. Each has strengths and weaknesses. Importantly, though, they are all at an early stage of development so picking winners is very difficult.
The scope of SAFE is more ambitious than most as it is attempting to replace three layers of the Internet protocol rather than one - and because it is not based on a blockchain. Maidsafe have been working on this for more than ten years, which gives them an edge on the newer firms in terms of experience and learning, and so long as it all works out it should be revolutionary. On the other hand, there are never any guarantees.
Ultimately if you have to choose one just pick the one that suits you best.