I wasn't trying to make an exhaustive comparison, but each individual contributes inertia to the market regardless of the total size of the market (see 1 below)

The MySpace argument often calls to the transient spirit of social media: one day it's hot, the other it's not. I recall Digg's death too after a controversial redesign. The issue with this argument is that:

1. Numbers not only increase inertia, but social products increase it too. If my friends aren't using a social platform neither am I, usually.

2. Facebook has such a rich culture of A/B testing that they'll never release controversial changes without a deeper understanding of the impact of those changes on engagement. They won't shoot themselves in the foot like Digg did with current leadership.

3. Facebook has a really successful history of acquisitions and its leadership is very aware of social media trends. It catches those trends early.

As a consequence of (1), I'd argue that Facebook has a tremendous moat in comparison to its peers: it has a large amount of users who are more likely to stay put. While Facebook also moves slowly as a consequence of (2) often enough, it can move really quickly on producing novel products.

Facebook certainly will face challenges in the future, but they're likely to be a lot different at this stage.