I think its a matter of quantity vs quality to a certain degree. I have established a level of trust with the slash RC site that FB will most likely never achieve. If I have a bad night or have a problem with another member on RC anything I post is then part of the running timeline...(I can not delete it). On FB it goes away if no one likes it or even worse, it stays forever because the person you had a problem with never lets it die.
on the other hand, I have used FB with great success promoting my veterans clubs charity runs and events. I used the FB ad publisher and ran cheap ads that target persons within 50 miles of my zip for Cape. Running a simple banner ad for slash for anyone interested in the hobby or any tag words we add like; fish, local, reef, yada yada...