It's tempting to suggest blockchain might be used for payments when Mark Zuckerberg had expressed a desire to look into cryptocurrencies. However, that's far from guaranteed. Outside of digital money, it's most commonly used as a decentralized, secure approach to establishing trust. You can verify someone's ID without having to go through a central service, as an example. Facebook could theoretically use this to check credentials, or even to trace user data collected by third parties.

If Facebook does find practical uses, it could be a while before you see any results. It won't be at all shocking if the company does something, however. Facebook is under intense pressure to keep a tighter lid on users' data following the Cambridge Analytica scandal, and it would only make sense for the social site to consider anything that would prevent a repeat incident.