I'd say that's certainly part of it. Even just escaping the hordes to stand out in a new place/way would be important to people. Think about how strongly teenagers define themselves with clothing and music to carve out a niche.
Beyond that, I think social networks are heavily reliant on active creators. If those people feel like minnows amongst the masses and grandmas, they're more likely to strike out in new territory whether it's a fresh network or mode of creating. Become a bigger fish in a smaller pond. Which leaves behind less-active users or people who've found reward/profit on the previous platform, making that platform start to feel more like TV than a mingling, social space.
Beyond that, I think social networks are heavily reliant on active creators. If those people feel like minnows amongst the masses and grandmas, they're more likely to strike out in new territory whether it's a fresh network or mode of creating. Become a bigger fish in a smaller pond. Which leaves behind less-active users or people who've found reward/profit on the previous platform, making that platform start to feel more like TV than a mingling, social space.