Facebook's (FB -1.8%) mobile ad quality is declining, writes BTIG's Richard Greenfield in a...

Facebook's (FB -1.8%) mobile ad quality is declining, writes BTIG's Richard Greenfield in a bearish note that's pressuring shares. Greenfield, who downgraded Facebook to Sell last month, thinks mobile ad sales could fall Q/Q in Q1 (a seasonally weak quarter), and that Street estimates for 2014 and beyond are too high.