FB is far from the only big tech company which has sponsored OSM as a competitor to Google Maps (Microsoft/Facebook/DigitalGlobe/Telenav/FourSquare/Craigslist have all sponsored OSM to some degree; Apple, of course, went its own way and created Apple Maps).

It's a reaction to Google Maps: a monopoly on high-quality up-to-date global maps with business location is dangerous to everyone else, as a chokepoint on mobile applications. It's less about 'acquiring data' and more about not being extorted by GM. Classic 'commoditize your complement' dynamics: https://www.gwern.net/Complement

Their motivation is self-serving but I don't think it's so nefarious. They use OpenStreetMap in check-in posts to display map data around locations that Facebook users have visited, so improving OpenStreetMap in turn improves the quality of this feature.

Training data from OSM volunteers, at the very least. Probably both cheaper and higher quality than e.g. mechanical turk.

Cheaper location data than they'd get licencing one of the other map providers.

They want to be seen to be doing something with computer vision/AI.

User location data is useless without the substrate of a proper map.

exactly my thought as well.

Whenever Facebook releases something we should all directly worry about what their real motives are.