If working at a company through and after its IPO period has taught me anything, it's that professionals choosing to put their money on the line or not is not really saying THAT much.
Facebook may become better at monetizing the remaining users, even as there is an exodus, and overall, it could show increasing profits and revenue. This would "excite investors" but it doesn't mean the company isn't screwed in the long term if things continue the way they are.
The other guy joked about Blackberry, because even as the US and Canadian markets were dumping Blackberry, the company continued to see some nice growth overseas, which also led to record sales, profits and so on. But it was obvious Blackberry was fighting against time with the iPhone destroying its core markets.
Most investors didn't see it at the time either, because they were just following the "positive quarterly results," much like Blackberry's leadership was.
If you believe that investors don't look at non-revenue metrics like MAU you are sorely mistaken. In fact, it's almost all they care about in this category. Twitter's stock is in the toilet because their user growth has tapered, whereas Facebook continues to grow at a healthy clip.
