Besides negative press are there any tangible indications of a large decline? Think about how many times you see a post like this on HN: "I barely use facebook. I just keep it for ".
Facebook started off as twitter, then added the kitchen sink. People thought they wanted the kitchen sink, but realised it wasn't worth the noise and/or privacy/security risks. Twitter itself has suffered 'feature creep', but to much less an extent. Facebook does have the 'monopoly' advantage, so I can imagine a lot of people will hang around out of inertia, but I also think that other competitors to very specific subsets will arrive, just as twitter did, and some will prefer to jump ship.