The routes from SFO that end in either Beijing or Shanghai are usually full from Tokyo (Where both have a change of planes) to those two cities because of the number of people boarding in Tokyo. Many people will arrive from other North American points and get to Shanghai or Beijing on the portion of 853 or 837 that is local to Asia.

Also, there are currently, much to my chagrin, no UA flights codeshare or otherwise between Hong Kong and Shanghai. The best you can do for comfort or service at the moment is DragonAir (a.k.a. Cathay Pacific).

Many of the peiople going into and out of Shanghai to/from Europe fly to either Beijing opr Hong Kong to get to Europe. Lufthansa, SwissAir, and Air France get you from Shnaghai to Europe. I imagine it is the rather substantial group that want to get to the UK but have to go through Paris, Franfurt, Beijing, or Hong Kong that Dickie B is after.