That "part of OSM" describes the good relationship the OSM community had with Mapillary up until this moment.

Also I'm quite sure if they hadn't "allow" OSM to use its images, their photo database would have been much smaller.

Facebook uses OpenStreetMap for many map displays so they may have an incentive to improve OSM and continue maintaining the good relationship between Mapillary and OSM. Facebook has contributed tools to OSM before, e.g. the RapiD editor [1], though I don't think they are used much.

[1] https://wiki.openstreetmap.org/wiki/RapiD

I'm not sure how this is related to the comment before but what do you think would happen if Facebook started misusing data of Mapillary for example? Do you seriously think OSM would "lock them out" or something?

Not possible. FB can use OSM data to build their own maps like everybody else does.

Ish. If Facebook violate the OSM licence, then that's illegal and something could be done about that. Though the OSMF would never be able to seriously legally sue Facebook. One side has considerably more money than the other.

But if Facebook abided by the OSM licence, but did things the OSM community really didn't like, the OSM project could shun them. Not accept sponsor ship, block FB's RapiD software from making map changes, eject them as a corporate member from the foundation, etc. They'd still be able to use the data, but they'd be a persona non grata.

So in short:

Nothing would hurt Facebook at all.

Exactly.

This is why this argument falls flat.

If FB plans to do shady stuff with Mapillary, they will and there is nothing anybody can do about it...