[Qz]

500 million vs 1.5 billion is a meaningless metric regarding which social network people will use. They will use the one their friends are using, and no one has near enough friends for those gigantic numbers to have any relevance.

[jlgbecom]

If it's such a useless metric, why is it always brought up as a clear example of Facebook's unmovable dominance?

[Qz]

It's a useless metric in regard to an individual person's decision to use facebook as opposed to something else. People don't care that facebook has 5 million users, they care that their friends use it. If all/most of their friends started using something else, they would too, regardless of whether that new network had 5 billion people or 5 hundred.