What would prevent the Chinese government from just blocking I2P?
Also, while Tor isn't used in China there seem to be some other Proxy Services that are used. A non-tech-savvy friend of mine who is Chinese citizen and moved back to China one year ago, uses some kind of proxy service to connect to sites like Facebook. I have the impression that the knowledge how to circumvent the firewall is rather common in China.
There are many paid VPN services available to those who have credit cards. These VPN services are based outside China. But this is quite expensive for most Chinese people to use.
The reason Tor isn't used in China is that most of the bridge relay IP addresses are blacklisted by Chinese ISP's and net-cops. Now the list of bridge relays are not public, but China has enough man power(working full time I might add) to get ahold of nearly all the ip's and block them.
Actually it wouldn't be too difficult to automate this at all.
Knowlege to circumvent the wall is available, it's the resources to do so that are lacking.
I2P currently is too small to show up on the Chinese governments radar. Apart from this I don't know how I2P gets the ip's of the darknet node. The only guaranteed way to be safe is to have a Freind to Friend network, with absolutely no public connection nodes.
Also, while Tor isn't used in China there seem to be some other Proxy Services that are used. A non-tech-savvy friend of mine who is Chinese citizen and moved back to China one year ago, uses some kind of proxy service to connect to sites like Facebook. I have the impression that the knowledge how to circumvent the firewall is rather common in China.