Social networks are notoriously vulnerable to the fickle tastes of teens and 20-somethings (remember Friendster?), so it's entirely possible that three or four years from now, we'll have moved on to some Twitter successor. But the key elements of the Twitter platform — the follower structure, link-sharing, real-time searching — will persevere regardless of Twitter's fortunes, just as Web conventions like links, posts and feeds have endured over the past decade.
Not all these developments will be entirely positive. Most of us have learned firsthand how addictive the micro-events of our personal e-mail inbox can be. But with the ambient awareness of status updates from Twitter and Facebook, an entire new empire of distraction has opened up.