I'm not very well versed in encryption but I know many here are and wanted to see if how I imagined this has any merit -

is this something where WhatsApp could encrypt messages word by word instead of by message, and keep a list of the hashes of top sent words and their counts somewhere, then facebook would send some ads directory to WhatsApp clients, where they'd encrypt each keyword in the dictionary, and then compare the dictionary hashes to the top send word hashes, and try to make a match on the client side for specific ads to show?

Or is this more that full encrypted messages would be scanned for some type of needle in the haystack that could infer some value about what's being sent?