TimOReilly points out that it now routinely pursues monopolies. That's it's only strategy. https://www.oreilly.com/radar/the-fundamental-problem-with-silicon-valleys-favorite-growth-strategy/
DaveWiner on how regulators should break it up :
Find the component of the company that really is open tech. Something that was open before they came along, that they foreclosed on, and used their monopoly to put everyone else out of business.
That's where you draw the line of separation. The core should be spun off into a new company that's well funded, with a charter to commercialize the tech while maintaining zero lock-in. Totally replaceable. Defined APIs that don't break.
If the company is viable with these constraints, great. If not, they have enough money to plan their own demise. The key thing is they cannot use their dominance to launch new products. Just the open tech.
Quora Answer : Social Media: Which social media sites are more likely than others to become irrelevant or extinct by 2024?
I was a heavy user of tribe.net around 8 to 10 years ago.
I'd say that's a fairly dying site. But it IS still around. And you can still see stuff I wrote there then.
By that criteria, I'd expect all the big sites in your question to still exist and have users in 5 years. I'll bet that they all have > 50% chance of being around in 10 years. And most of them will still be with us in 20 years. In fact, I think the volatility of social networking sites is obviously falling compared to the initial "Cambrian explosion" of the early 00s.
Some might get acquired. Tumblr is already part of Yahoo. And YouTube is part of Google. Pinterest, Quora, even LinkedIn, may potentially be swallowed up by someone else. Facebook and Twitter almost certainly not.
Google may, at some point, decide that G+ isn't a brand / identity worth defending. But there's no way that they'll give up on wanting to have registered users giving them personal / social data. So if G+ goes, expect it to be replaced by a rebranded version of something similar. I expect Google may eventually fold Blogger in to G+ so they become pretty much the same thing.
If Microsoft had any strategic sense (which they don't) they'd be investing in turning Skype into a social platform brand to compete with Twitter / Facebook / G+ by adding asynchronous messaging / photo-sharing etc. to the Skype client. It's not too late for them to do this, but they still don't seem to be moving in that direction.
Other of these sites are less generic social networking and more specific. I suspect these communities may retreat into providing more specialist services. Pinterest etc. and SoundCloud, Meetup, DeviantArt, GitHub will have to find ways to thrive by NOT being "social media" but through the services they provide members.
Quora might actually be in trouble, on the grounds that it's not clear what sort of business model they can evolve. Just "going big" and taking advertising feels very incompatible with the site's ethos and community, and I think there's a danger that such an attempt to monetize the site will damage it. (That's largely what happened with Tribe.)
Another important trend is that the big names you're seeing here will almost certainly get supplemented and challenged by a new generation of BitCoin / BlockChain / privacy oriented social network / market hybrids, whose feature is that they don't try to collect and resell your personal data. These services will rely on decentralization / P2P technologies rather than build expensive data-centre cloud infrastructure; and may use cryptocurrencies both to finance themselves AND as a barrier to entry / proof of commitment from the membership.