I don't see why not - use a relational database for storing long-term data, such as users, friendships, preferences, etc. and store ephemeral data such as locat

I don't see why not - use a relational database for storing long-term data, such as users, friendships, preferences, etc. and store ephemeral data such as location in a key-value store like Redis. Store summary statistics of the location data in your primary data store in scheduled background tasks. Just because you're storing a huge amount of one specific type of data, that doesn't prevent you from taking advantage of the features of a relational database.