They see slowing revenue growth, problems with monetizing mobile in an increasingly mobile world, and advertiser dissatisfaction (highlighted by General Motors' withdrawal of its Facebook ads on the grounds that they were ineffective).
Mostly, though, they sense Facebook Fatigue all around them. Many of Facebook's vaunted 900 million members are users in name only. Some have grown tired of Facebook -- how could you not tire of the insipid discourse that dominates the typical Facebook wall? -- and others only signed up under some form of duress (primarily peer and professional pressure and the fear of missing the cultural boat).
The bottom line is, it's hard to get excited about the future of a company and a service that likely faces a decline in interest and use. Buying shares of such a company isn't investing, or even gambling. It's throwing away your money.