Facebook Groups have not replaced forums because they are (more or less) forums. What is a Facebook group if not a less organized forum? Threads and replies, threads and replies. That's the basis of forums. The rest is extras.
Facebook Groups may be an alternative to hosting your own forum, for some. They are a different flavor. I don't know that they work for most people in that way, just because of how you are locked into Facebook and can't export your data or use your own domain name. That's a huge deal breaker for most serious online communities.
If both of those things were not true, they might compete a little more favorably with other choices, but in such a case they would only remain an option - not a replacement.