I think it's far too early to claim that Facebook is dead. However, the platform is certainly in decline and before long this haemorrhaging of users could prove fatal for the world's biggest social media website.
Personally I don't tend to use Facebook as much as I did a year or so ago, although I still think it's the best way to keep in contact with all my friends. The problem I've found with Facebook is that there is a considerable dearth of interesting or engaging content. Twitter is far better at displaying relevant content and in a far more streamlined form.
The rise of Google + must also account for some of Facebook's loss of users. I find Google's communities to be far more helpful and communicative than Facebook's groups have ever been. Google + also has the added benefit of having a hugely powerful search engine backing it!
Another observation about Facebook is that it has seemingly become 'uncool' with the younger generation. For some reason, kids these days have migrated in vast numbers to Twitter and away from Facebook. I believe this is probably because Twitter gives each user a better 'voice' and allows young people to connect with their famous idols far more easily than Facebook.
So, I don't think Facebook is dead, but they need to do something about this alarming decline, and fast!