On the heels of Tudou.com’s big round comes news that Oak Pacific Interactive has raised the gargantuan sum of $43o million in a round led by Masayoshi Son’s Softbank Corp. Oak Pacific Interactive (OPI) runs entertainment-focused community portal Mop.com, IT blog site Donews, the (probably moribund) video sharing site UUMe and — in light of all the buzz around SNS in the last year — their jewel-in-the-crown, campus-based social network Xiaonei.com. Xiaonei, not surprisingly, is the business unit that’s expected to see be the biggest beneficiary of this investment, and Matt Marshall’s VentureBeat even headlined his post on this “Xiaonei, the Facebook of China, raises $420M — better funded than Facebook.”

A press release from Oak Pacific and Softbank on Business Wire here says that the $430 comes to about 35% of the company. It’s not clear, however, how this investment has been structured. The Wall Street Journal reported that Softbank has ponied up “about $96 million” in early April, representing 14% of the company. That would yield a valuation of about $685 million. But the $430 million number for 35% would be almost twice that, at $1.22 billion. I’m trying to square this. The WSJ says that Softbank has the option of making an additional investment of $288 million in one year’s time, and that Softbank does intend to exercise it, but that takes the total to $384, and I’m not clear on how much the other investors — SBI (I’m guessing this is SBI Holdings, a fund originally set up by Softbank but which has been independent since 2006? The Softbank family is so convoluted) and JOHO Capital LLC– contributed to the round. Anyone have the inside scoop? In any case, the journalist who reported the WSJ piece from Japan, says the one who reported it from China, spoke with Softbank as recently as Wednesday afternoon.

A source knowledgeable about the deal tells me that the existing OPI investors have, by the terms of this new round, agreed to sell their principal to Softbank. These investors — DCM Doll, General Atlantic, Accel, Legend Capital, and Technology Crossover Ventures — kicked in some $48 million back in early 2006: what was then an astronomical sum, the money was earmarked for a roll-up of Web properties in China that critics said never really happened — except for the fact that they picked up Xiaonei, which they merged with their own 5Q.com, a competing campus social network.

Could the company possibly be worth this much? Immediate reaction among observers I know was that this is a big glaring neon sign saying Bubble!, but think back to what Masayoshi Son paid into Alibaba back in the day. He may have had to wait a while to cash out, but cash out he did, and handsomely — first with the Alibaba-Yahoo deal (remember, Jack Ma was given $1 billion in cash back in August 2005 to take over Yahoo’s business, while Yahoo took a 40% stake in Alibaba) and then of course with Alibaba’s very splashy IPO. Softbank isn’t a VC: Masayoshi Son can invest for the long haul. It might be time for him to pick a pony, and Xiaonei’s not a bad choice.

For Xiaonei to really take off, the site is going to have to undergo major reconstructive surgery — a procedure that it can now afford, it looks like. Xiaonei was built at a time when opening your API to third party developers wasn’t the sine qua non of successful social networks, and there is of course the site’s rather uncanny resemblance to Facebook.

This isn’t the last major round of funding you’ll read about here for Chinese social networks. For weeks now I’ve been hearing rumors of a very large investment in 51.com, a social network that’s immensely popular in lower-tier Chinese cities. One venture capitalist I spoke to said that on a fact-finding trip in Henan province, practically Internet user he talked to was on 51.com. Stay tuned.

For more about Xiaonei and Oak Pacific, check out Plus8Star’s Benjamin Joffe’s post here.