The reason for the jump: According to Reuters, Weibo is going private. And the reason for the retreat: Weibo says it isn't.
This morning, Reuters cited sources saying that Weibo's chairman is working with an unidentified Chinese state investor to take the company private in a deal valuing it at more than $20 billion. (The stock costs barely $13 billion today).
Clearly, that's a good reason for investors to bid the shares up, but no sooner had Reuters revealed these rumors than Weibo responded with a statement calling the report untrue. Chairman Charles Chao said that he has had no discussion with anyone regarding privatization.
That sounds pretty clear-cut, but it doesn't quite rule out the possibility that Chao was perhaps approached with an offer, nor does it categorically deny that he is hoping to take the company private at some point.
In the meantime, just the chance that Weibo could be thinking of going private seems to be enough to keep the shares floating higher today.