Just imagine combining Amazon and Twitter.
Another story on the amount of cash Alibaba is moving: https://news.ycombinator.com/item?id=5624887
However, it makes sense in that Twitter sees 4 times more posts a day, and has less rivals. But then Weibo fills the place of both Twitter and Facebook, so maybe still cheap...
And in this case, there are actually many other e-commerce sites and microbloging services here. Tencent's Weibo has more than 450 million users, for example. And Amazon.cn and JD.com are just two of the other big e-commerce sites.
(investment/ownership = 15% now, up to 30% later).