It's a suboptimal feature. Most developers like to write code that gets used a lot (at least I do). If a feature that you hoped will be popular is only used by a tiny subset, then you probably failed.

In Facebook's case, they may have written the initial lists feature as way to show they're "doing something" about privacy, so simply deploying the feature is a win.

But in Google's case, they've made Circles a central feature of their platform and it's coming up short (so far at least).