Some additional commentary on the subject:
The way Facebook would spread in China is through "your friends having it." It's exactly the same way it spread in the US: your friends at another university have Facebook. They talk about using it, and you want to stay connected, too. When Facebook opens up at your school, you quickly get an account so you can be in the loop along with them.
In 2007, we saw this effect happening at Peking University and at Tsinghua University (and probably elsewhere) among Chinese students who had made friends with international students -- virtually all of whom were Facebook users. (We definitely saw this happening among FACES delegates. =D)
This highlights one (really important) thing that Facebook has going for it, which Renren et al. don't have: that connection to international users. At the point Facebook re-enters the China market, given how things go in the next few years, that might even be a connection to the rest of the world's users.
Maybe it's only a small subset of the population in China that would be interested in this -- the educated elite at the top universities who have the opportunity to even know international folks. But that's how Facebook started out, too. If the option to use Facebook were available, that elite demographic is going to stay engaged. They eagerly read foreign news and have discussions with their friends abroad. Facebook would be the perfect tool to complement and strengthen those connections; and once they're on board, their more inland-oriented friends may follow suit. (After all, many tides in Chinese history are led by students and intellectuals.)
Now as many folks have pointed out, there are a host of local competitors already filling the "social" niche. Maybe these services will be so deeply integrated into Chinese users' lives that people won't want to leave those other ecosystems. "I've achieved level 499 on Happy Poodle Farm ... I have a prize-winning pack of canines ... I can't leave Fluffy behind, I just can't!!!" Or you might have regional networks that are deeply embedded in a certain social networking site. (I assume geography matters because your "friends" are more likely to be in your town, at least initially, though maybe that correlation will erode a little as mobility increases). Some of this represents only inertia (not to be underestimated) and some of it represents actual integration -- ganglions you'll have to carefully prise apart and free up, or move wholesale as a group.
So perhaps you'll end up with what you have today: because China is a big enough market, with many (and still growing) numbers of Internet users, many different social networks can live and coexist there, serving different demographics, depending on their level of international connection, and customized to the different experiences they want.
Now a caveat: a reverse flow is going on, too. Your Chinese friends use "Xiaonei", so why wouldn't Facebook users migrate to (ahem) indigenous Chinese sites? I did sign up for a Xiaonei account back then, primarily just to "see the other side."
But the only reason that I use it now is to keep in touch with a handful of students in the Tsinghua Symphony. They aren't on Facebook because they claim that it's kind of a hassle to regularly leap over the Great Firewall (繙墻). I kind of grumble about it every time I go on Renren -- using Facebook to connect with my friends in most of the world, and then having to sign on separately to Renren to stay connected to those few Chinese friends. I'll do it, but I'm not happy about it.
[This brings up a related point: overseas Chinese students -- people from the PRC who go abroad to study -- maintain their presence on Renren when they're in the US. They regularly roam the site and post photos to it, and perhaps they may not have the same qualms about using both. (To be honest, I don't know if it's by choice or by necessity for them, but I shall ask around). But again, that's because everyone at home has no option other than a domestic Chinese site, like Renren. If Facebook were available in China, then the dynamic could be different: might other people sign up for Facebook to stay in touch with them?]
I also have to duplicate my posts, first posting in Facebook, then translating into Chinese and posting on Renren. To be sure, half the stuff I post on Facebook I don't even think about posting on Renren -- cultural sensitivities, you know. =P And when I do post things on Renren, half the time it gets summarily deleted anyway, probably because I've used words on the sensitive list. (Nobel Prize/諾貝爾獎, anyone?)
But here's another factor that could work in Facebook's favor: if Chinese users were to have both Renren accounts and Facebook accounts, they could see the censorship happening right in front of them, side-by-side. Facebook comes off looking way better in that comparison. (I admit, this point is moot if Chinese users never even think about posting on political topics because they don't find it interesting, unlike entertainment news and pop star gossip. That might be a somewhat different problem).
Still, if Facebook ever entered China on its own terms (i.e. not censored), some kind of integration would probably happen in the end. We all love having choices, but we also prefer having an integrated service, unless there's a good reason to keep spheres of our lives separate. ("Corporate" vs. "Personal" networks, "Work" vs. "Play" -- though I guess Facebook is trying to work around that, too.) I doubt Renren would willingly allow you to take your data elsewhere, but I'm sure some enterprising app developer could make a "Profile Transitioning App" that would bundle up your data from Renren and help you port it to Facebook. That could substantially lower the barrier for transitioning. And independent app developers would probably be happy to release your data, because they'd want to jump on to as many platforms as possible while retaining users.
Maybe you can save Fluffy after all.