Traveled solo past 15 years, was in China (Beijing only) in 2006. Have also traveled with tours a bit and with others, but have strong preference for solo now.
The biggest difference between solo and not solo is you need to pay a bit more attention to your social needs, and you have much more flexibility.
Best way to get social interaction going is at your place where you are staying. Smaller places, hostels (can be luxury), that sort of thing have a natural way of getting interaction going. Same thing with booking bus trips and such for day tour activities, that can usually be booked from your lodging and you'll be hooked up with others. Hostels specifically aren't just for 20-somethings anymore, I've met ample of 60+ folks.
If you want to visit the great wall for example, book a bus tour with other westerners, you'll have your talk for the day. It'll be more expensive, but might be worth it. I've also given rides, sometimes long distance, to other travelers (in New Zealand for example), that sort of thing. I wouldn't recommend renting a car in China though, but you might be able to share a taxi with fellow travelers.
China specifically: good to realize in certain areas you might be the tourist attraction, and in the minority in some places. The bulk of tourists in the forbidden city for example (>90%) are Chinese from other parts of the country. Not all of them have seen their fair share of non-Asians yet.