Singapore have been named the most start-up friendly country in the world several years in a row.
You can easily "replicate" as you put it, the conditions US start-ups have.
The main factor behind those highly successful web-based companies you mention is they have received a huge financial backing through funding.
Now the core shared factor of the companies you mention is they target a very broad audience, which have nothing to do with their success but rather have been a reason behind them getting the funding they needed to grow, when they started attract attention from investors.
It's a global market, saying that the US population have something to do with the companies success I believe is invalid. Anybody can target the US market no matter what country they reside in.
What makes the US stand-out with their web start-ups is they do have a large amount of big funding companies and of cause because the US have a huge population it of cause means even with a small success-rate for their businesses there will be some big success stories.
If you do a little digging you'll find there's a lot of big web-based companies residing in Singapore and many means to get help and advice as a start-up.