How do I replace the Xenforo Facebook recommend/like logo with my own?
Upload your logo to the server and change the image path in the ACP -> Styles -> Style Properties -> Header and Navigation -> Settings: Facebook Open Graph Logo. Due to caching it can take several weeks for the image to update on the Facebook servers. You can attempt to force an update of the cache using the debugger.
Note that the image must be square with minimum dimensions of 200px and maximum dimensions of 350px.