Snap(+10%) rallies after long time skeptics Greenfield labels Snapchat a "Buy". A 10% price swing simply from this seems like an overreaction to me. Just curious what the community thinks?

I think so too. But US market has traditionally to be proven to be the more volatile market compared to other market. Like how Benjamin Graham puts it "In the short run, the market is a voting machine. But in the long run, the market is more like a weighing machine.". In the short run, the market will always be driven by sentiments, either overly optimistic or overly pessimistic sentiments.