Recommenation Question

How important is it that your recommendation letter comes from someone you know very well? Do the sororities really care or is it just like a formality? My mom heard on some parents page that its not ok to ask on Facebook what sororities I need recommendation letters from but I've also heard that it's ok to do that so I'm confused. Of course I'm getting as many recommendations as I can from people who know me really well, but if I can't find someone I know very well does the rec count less and I might be dropped? Thanks!