Google and Facebook may have a larger user base but it is not the in the same realm, e.g. I use iMessage to talk to my family, my close friends, parents from my daughter's preschool, co-workers, people I've only interacted with professionally - vendors etc - all it takes to connect is a phone number which is universal, and like you say it falls back to SMS. Compare that to Facebook where you have to add someone as a friend - which I don't do often because it just feels odd, or Google where messaging ecosystem is a bit fragmented with Allo, Duo, Hangout, Hangout Chat etc. Signal and WhatsApp are actually viable options but compared to iMessage feel heavy and not native, however I do use them both.

And like you mentioned - spam - I have yet to receive an iMessage spam message. That is pretty amazing.