What we're seeing the social networking space at least -- and this is really interesting -- is a verticalization of the social networking space.
Over in the physical world, if you're a new company and you want to produce, I don't know, refrigerators, obviously you aren't going to start from raw materials and try to recapitulate the industry of forging metal, printing circuits, etc. In fact, you'll probably buy the vast majority of your components in a pretty late stage -- like, unless your special sauce is some new pump design, you'll probably buy your pumps fully formed.
Similarly, if you do think of an awesome pump design, you're more likely to try to sell it to existing fridge makers than also take on the design and creation of the entire refrigerator. And clearly this is all well-understood and unremarkable.
I think that what's happening in the social networking space is that there's a realization that there are (at least) two very difficult stages involved in creating a successful company. First, you have to build a gigantic userbase, fighting entrenched competitors with network effects, etc. Then, just as difficult, you need to monetize those users without driving them all away.
So, just like the pump company builds pumps and sells them to refrigerator company with no aspirations to build an entire fridge, we're seeing companies like Snapchat, Instagram, and WhatsApp tackle just the userbase-building. And then they sell to Facebook or Google, who have a lot of tools to monetize a userbase.
Of course, unlike the pump company, which just sells a product, the userbase companies sell their entire company. But I think it's fundamentally the same thing going on here: you can be great at building userbase without deceiving yourself into thinking that you're also great at monetizing your userbase. So your business model becomes, "Develop a giant userbase, then sell to Facebook or Google."