Later, it occurred to me that this might be a function of the number of people one has friended on Facebook. I have all of 34 friends on the Facebook account corresponding to my name here, and I fairly reliably see most of what they post, although there are occasional dropped posts. It does help when I use the "most recent" option, but I still don't know if that gets them all.
Facebook's algorithm for showing posts on one's feed is obscure but it does seem to try to reduce the number of posts one gets to what they consider a manageable number. Evidently they have decided that simply forwarding all the posts one's friends have written is overwhelming for people with many friends. Overwhelming the advertisements, no doubt.
In any case, if I needed any more reasons to stick with LJ/dw as my primary platform, this is yet another.