I think Dropbox is a great example to illustrate the weaknesses of the product-to-feature template. Yes, a product can be crushed by a feature, but only if that feature is "good enough".

Dropbox, in particular, is remarkably resilient, because the competing features still aren't "good enough". Apple and Microsoft are actually relatively weak competitors because neither of them "plays well with others" (and having access to your files "anywhere" is a big part of Dropbox's value proposition). Amazon is a more significant threat (competing with your infrastructure provider is never easy), but even they haven't done enough to make Cloud Drive simple and ubiquitous.

Looking at Google+ that way, I think many products won't be threatened by Google+'s features. For example, Google+ doesn't support anonymity the way Tumblr does, so it isn't likely to be a "good enough" replacement.

Facebook might be special case, though. Other than the existing user base (which isn't an impossible hurdle), I don't really see any dimension along which Google+ isn't likely to be a "good enough" alternative. On the other hand, I've never been a fan of Facebook, so I'm probably a bad person to assess that.