I have read somewhere speculation that Jane Austen might well have read that book (published in 1782)or heard about it from her cousing Eliza de Feuillade. I will try to find the reference for it tonight. I've linked below to an old L&T archive thread on the topic, but I also read it in a biography of Jane Austen--possibly Claire Tomalin's. The characters in Lady Susan do seem to act quite a bit like Valmont and the Marquise de Merteuil at times.

I agree that Willoughby is not of that ilk--the calculated evil and pleasure to be derived expressly by conquering and destroying someone. However, the kind of selfishness and cold-heartedness he demonstrates does not, IMO, make him a whole lot better than they are. Just because Willoughby didn't actually care about them enough to worry about what effect his actions were having on them, young women who are used and cast aside as Marianne and Eliza (and possibly others) have been are not really much better off.