Thinking more about the acquisition of Whatsapp and Viber, it strikes me that there are three phases in the evolution of these kinds of products (so far).
The first phase was about price and the PSTN. The cost of roaming and the cost of international voice bore little connection to the underlying infrastructure cost, and a host of startups emerged to try to arbitrage between high list prices from mainstream telcos and the low underlying cost, using the data channel to go 'over the top' (OTT).
The obvious winner was Skype, which unlocked vast demand for free voice and meaningful demand for actual paid international voice. Skype upended the calling card business, which did the same thing but on a smaller scale with a worse UX and a different route to market, but had very little impact on mainstream telcos, since international voice, though high margin, had never been a big part of their business - most people don't know anyone abroad. Skype was the big success but there were many other companies trying to do things in this space around roaming and international, few of which became meaningful businesses despite often doing some technically very clever things.
The second phase came with smartphones (Skype wasn't really mobile to begin with), and is partly also based around arbitrage, in this case between unmetered data plans and metered SMS. BBM (not entirely intentionally) was one of the first but a swarm of other companies (at least 50) offer free or near-free text messaging.
This is a challenge to telcos, who have found splitting out SMS and selling it separately a convenient marketing tactic (when I worked at Orange the pricing department was called 'Value Based Marketing'). Hence, mobile operators must now rebalance their tariffs, rolling data and SMS (and indeed voice) into a single monthly plan - this poses all sorts of transitional problems but the actual financial impact varies wildly by country - see here for a discussion.
Once the telcos have gone through this process, pitching an app on 'free SMS' is not much of a pitch. However, most of them do more than this, adding anything from group chat all the way up to the full-on content distribution platforms of Line or Wechat (discussed here). What they almost all have in common, though, is that they use the PSTN numbering system but never connect to the PSTN. That is, they look at your phone book and use your phone number to identify you and see which of your friends have it, but they don't actually make phone calls or deliver any SMS. So for these apps the PSTN is a social graph but not a piece of infrastructure. Add things like home-screen icons and push notifications and one sees that the smartphone is a social platform, in a way that the desktop web never was.
In other words, Skype's strategy was all about the PSTN but Line or Whatsapp effectively ignore it (though Line does do some bundling deals with MNOs in emerging markets).
I think, though, that there's also a third phase. Why is it that if I use a cab service app I can only talk to the driver by a PSTN voice call or SMS? Why does a restaurant booking app send me out to the phone dialer? Isn't it inevitable that at some point they'll bring that loop in-house? Shouldn't that, also, become part of the generic data layer rather than the telco's voice or messaging layer?
Moreover, why should it only be explicitly social apps that access the phone address book to find my friends? Why shouldn't a retailer's app tell me that I have 8 friends using it, and let me share products with them rather than emailing them dumb, untracked URLs or even, quite probably, screenshots of my smartphone with the app open?
In other words, WhatsApp, Instagram and a dozen others have unbundled Facebook, but at what point and in what ways does WhatsApp itself get unbundled?