Flickr may have lost 63% of its photos after being acquired by SmugMug (updated Oct. 2019).
This chart shows the number of public photos uploaded monthly to Flickr. It is based on the count of photos uploaded every hour since Flickr was started in 2004; this data is then consolidated into monthly figures. Therefore, photos that were uploaded but deleted later on are not taken into account. Assuming that users only marginally delete photos, the total number is likely to be a close approximation of the number of photos ever uploaded. This is represented by the blue curve.
However, it seems that this assumption became wrong at some point in 2018: the red curve still represents the number of photos uploaded monthly, but with data obtained in July 2019.
What explains the severe drop between the blue and red curves?
From discussions with other users, this is likely related to Flickr being acquired by SmugMug in 2018 and the new account policy set in January 2019: free accounts are now limited to 1000 photos instead of 1TB previously, while the yearly price of the pro subscription has simply doubled (!!!). Such changes are likely to have made some free- as well as pro- users delete their photos and leave Flickr. Yet, it is unclear if whether deleted photos of free accounts beyond the 1000 limit or if they are just hidden.
If this is indeed the main reason for the drop, it means that the new SmugMug policy has made Flickr loose 63% of the photos ever uploaded: the total number of public photos uploaded until Jan. 2018 was 6.47 billion as measured in early 2018, it has decreased to 2.38 billion as measured in July 2019.