At the risk of nitpicking, I think if we want to understand Twitter and their success, we need to differentiate "platform" from "ecosystem." And we need to take a look at the 1.5 million apps accessing Twitter's API, and how different they are from the apps within Facebook.
I'll bet that 96% of the "apps" under discussion have absolutely nothing to do with Twitter. These are "apps" like newspaper web sites that give a button to Tweet an article you read, GPS apps that give a button to Tweet your location or something you just saw, other social network sites (like LinkedIn) that give a button to Tweet a link to something you post on that other system, and so on. I say that these have nothing to do with Twitter because they're all "apps" (or "sites") that do something else altogether, and connect to Twitter to TRY to grab a quick and cheap PR jolt. They're using Twitter to get people to come to their site, and are not inherently leading users to Twitter.
Why do I say 96%? I'm guessing that 50,000 apps might do something that's truly centered around Twitter, but not more. Pure hunch.
But if this number is even close, I think it's a hugely mixed message about Twitter and its so-called ecosystem. On the one hand, it's the system that everyone wants to throw as much stuff as possible, and correspondingly it's the system that everyone wants to troll to find the few interesting nuggets. This makes it the ecosystem, or maybe more accurately "platform," of choice to note whatever your app or site does.
On the other hand, I'm concerned that there's no loyalty or lock-in for this 96% of Twitter API users. They could just as happily throw their notes onto FB or G+ or any other platform, and most of them do that. To a news site or another site that Tweets what its users do, Twitter isn't an ecosystem that they're operating in, it's a place to throw notes along with all the other places they throw notes. The same is true for the information trolls (like me) - I'll troll for information anywhere, and I do, and I'll stick with Twitter as long as their quantity stays high.
(Also on the other hand is the danger that these 1.45 million apps and sites are adding noise, not signal. When I scan Twitter, I often see about 50 Tweets for the same link or article, and that's a minimum. More apps enabling more people to post the same thing will eventually be a bad thing.)
Contrast this with Facebook, which has a true ecosystem for apps that don't only post or process Facebook content but live and operate within the Facebook ecosystem. This is a whole different level of ecosystem. An app (however silly it may be) that is accessed directly by Facebook users on the Facebook site, most of whom couldn't find the app's own web site if it bit them on the leg, is locked in to Facebook in a way that very few Twitter-accessing apps are. This makes Facebook not just a platform for content but also an ecosystem which locks in both users and the apps that truly live within that ecosystem.
All this is why I love Twitter as a platform to post on and to troll for information, but also why I think they're a much less healthy ecosystem, and much less a true ecosystem, than they say they are. As long as they can remain effective for both sides, making it worth Tweeting and worth looking for Tweets, they'll do well as an information platform and presumably make some money selling Tweet feeds. And they have a much bigger opportunity to sell analysis of Tweet streams. But it's a very precarious place to be, unhealthy in the sense of having very little lock-in, and one that will be hard for them to leverage for anything beyond Tweet feeds and analytics.
So what's Twitter's future? I have doubts about advertising because the biggest Twitter users don't use the Twitter site or app. They should do it, because they'll earn something and why not, but it's not a business model that inherently fits their platform.
What they should do, I humbly suggest, is invest hugely in the area of analytics, filtering, selection tools, and other ways that professionals can buy information that Twitter is a strong position to give. Remember when Google claimed that Google searches predicted flu epidemics? What's true for Google in a week-to-week basis is probably true on Twitter on a minute-to-minute basis. Throw in location tagging and they're sitting on a gold mine of mineable data, that they have better access to than others.
In the next 6 months we'll see Twitter using some cash to buy a few analytics companies. For sport, let's say one will be location-based, one will do real-time text analysis of tweets, one will analyze links in Tweets, and one will analyze pictures linked to in Tweets.
At the same time, they'll continue to promote any and all sites and apps to use their API to Tweet automatically. Even without profiting on this, they'll maintain their status of the repository of real-time activity.
In the end, being an ecosystem may be over-rated. I'll take a platform any day, if it can tell me what I want to know.