Photos on social media are stolen and copied.
In my case, with huge watermarks, just like from agencies, but their watermark is smaller.
Photos which have been legitimately bought and are on the web are also stolen and copied, but without watermarks they are attractive to a much larger number of non-buyers, and these are usually
much bigger than I post to SM. I find
much more of that sort of theft than 'lifts' from SM. YMMV.