Sina Weibo passes 500 million user mark, how's your site coming along?

Remember those halcyon days last November when Sina Weibo passed more than 400 million users? Well, a mere three months later, and the microblogging site can now boast that it's got 503 million compulsive over-sharers. To put that figure into perspective, were "Chinese Twitter" to be its own country, it would be the third most populous nation on Earth. While it should be celebrating such figures, there's a few signs of trouble on the horizon, as local rival Tencent's WeChat hit 300 million users last month -- not to mention some disgruntled former users inviting people to switch to Twitter.