Facebook's User Growth Continues to Decrease while Google+ Adds 625K Users Daily
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If you are likely to be on facebook, you probably already are. I don't think it's surprising the growth is decreasing.
I would be (pleasantly) surprised if the growth was negative, but that's not going to happen :P
http://www.insidefacebook.com/2011/06/12/facebook-sees-big-t...
I think when your core users and initial early adopters are leaving your product/service, you need to start worrying immediately, rather than wait for that 3-year late "financial impact" to "believe it" that you're in trouble.