For every "social media is changing our world" moment that pops up every few months there are 12,460,000,000 worthless tweets clogging up the other 89 days.
It seems to me that the primary use of the service is to provide an IV drip of useless information for update addicted. The "great" moments are purely incidental.
The trick is to follow your interests. If you don't care about world news, NASCAR, or celebrities, don't follow them.
While Twitter is the current market leader (imo mostly due to constant hyping by mainstream media who apparently feel "tweet" is of analogous magnitude as "email"), it could easily be replaced by any number of extant services, let alone potential future services.
Does this matter at all? Everything could be replaced by another service. Nothing is sacred. But what really matters isn't what will come, but what exists. Twitter exists. That's why it's used. I'm really not seeing your point.
Google has competitors, but they can't just upset Google overnight because it is really hard and takes a really long time to build a search facility of equivalent utility to the one Google provides. While some people challenge them, Google is still the objective king in searches, and companies with a huge amount of resources, like Microsoft, struggle to supplant Google in the quality of their offering.
Facebook is a very complex system with a lot of support for a lot of different things. Facebook is more in line with Twitter in that it's strongly dependent on network effects, but Facebook requires investment of significant time in building relationships and composing a profile; they have some reasonable sense of lock-in here. And if you've looked around, it is still difficult to develop a platform that does everything Facebook does (or more), especially with Facebook's rapid deployment of analogous features anytime a competitor tries to one-up them. They copied and thereby neutered the viability of Circles lickety-split and Google+ could no longer claim a significant feature advantage for those concerned about privacy.
Twitter is just a stream of short strings, easily scrapeable and convertible. The site can be cloned on a conceptual level in a couple of days. Twitter may have done some serious scaling work, but in the end they have no unique offering, no killer feature, nothing that a normal developer would have difficulty or serious effort duplicating at the conceptual level. Scalability of course takes time, but it's highly implementation dependent and occurs as necessary; Twitter's internal performance is not a serious user-facing barrier or feature. In fact, Facebook and Google+ already include all the functionality of Twitter.
There is no reason to stay on Twitter other than pure network effects.
Given the massive role Twitter played in Egypt's protests, I wonder if such a thing would still be possible given Twitter's current corporate philosophy. And the fact that I even have to wonder that, now, is why we're blaming Twitter.