Sure, spam is a big problem, and that's why we've invented spam filters. Google was able to do that and it works well. Granted, Google is a huge company with lots of resources, but so is Facebook, so why can't they be bothered?
>Don't use it to message strangers, it's very explicitly not what the platform is designed for.
Wrong, it's designed to help people get in contact with each other. This doesn't mean everyone wants to be friends first before exchanging some messages.