Why did MySpace fail? Users are known to be sticky - they tend to stay with the same newspaper, brand of chocolate, chewing gum, household cleaning products and so on. But they are only sticky up to a point. If something is better by a sufficiently significant margin, then they will switch (or some will). You can keep those customers just by just matching your competitor's improvements. For many things, the pace of change is reasonably slow. Also, many people just like that specific taste (even if they don't think it tastes as nice - like Coke and New Coke). There is some change (e.g. sugar-free gum), but it's easy for the leaders to keep pace. Perhaps in some cases, the scope for possible improvement is less than the margin needed for users to switch - so you ever fail.

MySpace had that huge advantage of many sticky users, but they stopped improving it, and a competitor (Facebook) made an improved experience.

Twitter is safe for as long as there isn't a sufficiently better competitor - "better" being in terms of the benefit to users, not any engineering quality in itself.