I believe its of no use, in fact i don't even invite my friends to like my page unless i feel they or their circle are related to my field. Some of my facebook friends are also my clients (i am a copywriter) and i often get request from them to like some page(Which i hardly do unless i have any reason to do so). At the end of the day the only good that can come from unrelated facebook like is a small chance of empty traffic back to your site, provided you had your web link posted on your page. But this is of little use and could even increase bounce rate of your website since these people have little or no reason to check your website.