This is what DDG does and they're not exactly rolling in dough. Also there are a plethora of contexts where this wouldn't work well. For example, Instagram has

This is what DDG does and they're not exactly rolling in dough. Also there are a plethora of contexts where this wouldn't work well. For example, Instagram has the best targeted ads I've ever seen – basically every ad I get is for something I would consider buying. I don't think their ads would work nearly so well if I got an add for a guitar just because my friend tagged her post with #guitar.