Filtered or not, US web companies don't tend to do well in China. Amazon and eBay aren't filtered, for example, but they both completely flopped here. A lot of it has to do with US tech companies not understanding how (or being unwilling) to do business like Chinese.
Google was the one that really invested in China and though not in the same league as Baidu yet, it was definitely moving up in market share until the government turned hostile on them. They definitely "understood" how to do business in China: they had a big Chinese staff, they even had a music search like Baidu which surprised me because it seems like something RIAA would go nuts at, and they made their own very good pinyin input system.
I'm guessing if Amazon or ebay got successful and competitive with their players, China would suddenly notice some pro-Tibet book on there and use it as an excuse to give them the boot.
eBay are famous down here in New Zealand (at least with those that care) for believing that serving a dial-up market with heavy picture laden HTML from 200-300ms away and no local presence was going to work. A local start-up (Trade Me) destroyed eBay pretty quickly and has gone on to post better per person numbers than (I believe) any eBay affiliate. Same story in China. I say there are/were too many MBAs in eBay and not enough (especially local) tech-heads.
Meanwhile Skype is doing great things post eBay ownership.
L an ex Trade Me MBA. The only one at the time, and part time at that.