"masternodes are vulnerable to sybil attacks, especially since it was instamined."

 even if one person mined/bought 2,000,000 coins there would only be a 0.35% chance of deanonymizing a privatesend transaction. so that's over $26 million (at current prices) they would have to put up/risk losing just to sybil/snoop on less than half a percent of all transactions.

probability chart
http://imgur.com/WuooipA

#2
http://imgur.com/pRkkyrh

chart from dash whitepaper
https://www.dash.org/wp-content/uploads/2015/04/Dash-WhitepaperV1.pdf