From Morningstar's Yue Yao, in his May 11th note on Weibo:
"... we have a downbeat view for Weibo's long-term outlook for three reasons: First, the content censorship imposed by the Chinese government restricts the flows of information on Weibo platform. Secondly, Weibo's user base is geographically limited to China, so user growth could reach saturation earlier than Twitter; Finally, we believe the display ads on Weibo are less accurate and of higher volume, which lead to an inferior user experience."
Here's an analyst who doesn't pussyfoot around the censorship issue. I don't think censorship is a big deal for most users, frankly, but it's a balancing act for the government -- they don't want to kill Weibo completely. As far as user experience goes, most Chinese love clutter.
Weibo will report earnings tomorrow (May 21); it'll be interesting to see how much ad revenue they've grown and how much of it comes from Alibaba alone.