It is. Weibo doesn't do well outside of China, it is not international like Twitter is.
> that PayPal is the only really huge payments service
From the perspective of a non-China resident, that is definitely true.
> Amazon and eBay are the major players in ecommerce
Ditto. Though Taobao is trying to become a major international player.
> For any service you can think of in the US, there's one in China that might be even bigger.
Except the services in the US are mostly international. The services in China are almost always limited to just China. The fact that UnionPay is the largest ATM network in the world (by users...almost all in mainland China) doesn't mean diddly when I visit the Netherlands or India where my ATM card is USELESS.
China has decided they can lock out international internet services (via the GFW) and build their own isolated network. Internally, it is huge, but the isolation cuts both ways.