Of all of the social networks I use regularly, I think that Twitter is probably my favourite. It’s the least constrained, most chaotic. It’s the place where one can interact with friends, strangers, people, “brands”. It’s kind of lawless. And as a result, it’s not to everyone’s taste.
My hunch is that, for the way in which Twitter promotes social promiscuity and that’s what makes it what it is, it’s probably reached its natural mass audience. Because although there are quite a few of us who are happy making new connections across the ether of the interwebs, most people aren’t. Most people who use social networks use them to connect with and keep in touch with people they already know. That’s how most people, it appears, use Facebook. That’s how most people are happiest.
But of course “slowing growth” or “no growth” are unacceptable in the bullshit world of Silicon Valley tech funding. It’s all 10x this and exponential that and hockey stick the other. And that is the dilemma that Twitter now faces. To make it safe for a bigger mass market (like the 300m it current has isn’t enough) it will need to make itself more palatable to a mass, connecting with people they already know, audience. And to do that it will break what makes Twitter the unconstrained, chaotic, socially promiscuous place that it currently finds itself.
mmitII 
As the haystack grows bigger, finding the needle becomes ever harder, so I suspect that will be the enduring challenge for Twitter. With things like trending topics, they already do this ‘reasonably’ for a mass market, but I suspect other platforms will do a better job for more niche fields.