The shareholders control the board, and the board controls management. Thus, unhappy shareholders will mean new management. In FB's case however, Zuck still co

The shareholders control the board, and the board controls management. Thus, unhappy shareholders will mean new management. In FB's case however, Zuck still controls (through proxy) the majority of votes; Zuck cannot be ousted. There are other problems with a non-performing stock too; employee morale may be (and is increasingly?) tightly related to the share options they own. If management is not performing well and costing their employees money (!) that can ruin morale. This doesn't just involve Facebook; currently the market is pretty bullish on web companies. It can quickly turn bearish, restricting the capital that currently flows so free.