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.
No matter how many places Google pushes G+, people are not going to switch unless they can achieve the same functionality by copy-pasting a link.
Consider, however dubious I might find Facebook, the only thing that I would move to is something like what diaspora promised - a less corporate, more anonymous platform. I won't move to Google+ no matter and so some number of my friends won't find me there ever. A small number of my friend have talked about moving to G+ yet are still on Facebook. I suspect this dynamic will protect Facebook until something offering a more desirable platform comes along - G+ calling itself an "identity platform" seems to scream that Google will be selling its members even more than Facebook sells us.
If any app being social means being popular among general web users than yes Google Docs / Office document is not make-or-break feature for social. But when you use Google docs within an organization for team collaboration there is no better tool than this I've found so far. I doubt whether they are working on Google Docs desktop or not but I think that will be a one on one competition to MS Office.
>> (in fact, my favorite Google office productivity product, EtherPad, was killed in favor of Wave)
Etherpad was simple tool. Yes, we can't run it on etherpad.com, does it means they killed it ? I don't think Google killed it, they opensourced it. There are plenty of other hosted options available and are linked at the same domain http://etherpad.com. Why don't use them, they run Etherpad as well.
My longterm strategy is to use different companies for different services. It is a slightly bigger hassle than to have everything integrated by one company, but not that a big hassle, after all. My goal for the future is that Google will own only my search strings, and not much else. I would even like to replace Gmail, but didn't find an alternative yet (but live.com looks promising).
Most users don't care. They're not thinking about the wider implications of Google having all the information Google+ will accumulate. That's completely incidental to their goal, which is to keep up to date and share stuff with their friends. And they want that to "just work".
They want convenience, which means not wanting to have to get past obstacles like logging into a completely different site to share a file. Even if the UI Google offers is a bit more sucky than the competition, e.g. dropbox, not having to go to the competition in the first place can easily be enough to make the Google experience the overall winner.
If they think about the "eggs in Google's basket" problem at all, many will see that they give Google all their personal info (name, dob, email address, location?, etc...) to use Google features, and they give Facebook all the same personal info to use Facebook features, and they give some other company a subset of their personal info to use, e.g. file-sharing features, etc.... Seeing as how Google already has the personal information they think about when they think about personal information, it won't occur to them that the meta-information Google will get from tying together. Rather, 3 or 4 companies have their personal information now, and they could reduce that to 1 by doing everything with Google.
That said, the challenges will be many, not the least of which is how do you organize the user interfaces (plural because of mobile vs web vs other devices) on such a super product and not end up with lukewarm soup.
I think the author is drastically overstating the inevitability of Google's product superiority. Google already spectacularly failed once at messaging with Google Wave, and Gmail doesn't actually have the market share that most people think it has (even just considering webmail). Office documents don't seem to be a make-or-break feature for social, and Google Docs has had very limited success competing with Microsoft Office (in fact, my favorite Google office productivity product, EtherPad, was killed in favor of Wave). Spam filtering is legitimate, but it's not that hard of a problem really in a technical sense; Facebook could keep up. Video chat is a weak point for Facebook, granted.
Besides, integrating your competitor's product as a feature of your own doesn't always spell success. Microsoft integrated Bing with its operating system through the IE search box, and look how little that has mattered. Both Apple and Microsoft have tried to "feature-ize" Dropbox, and Dropbox seems to have emerged unscathed. Instapaper is doing fine even in the face of the Safari Reading List. The list goes on. (Sometimes the strategy does work, such as IE against Netscape, but it's hardly clear-cut.)