Despite the accepted answer above, Tolkien did leave the merest hint as to the possible fate of the Entwives, in a conversation between Sam Gamgee and Ted Sandyman in the first chapter of the Fellowship of the Ring, Sam asks:
But what about these Tree-men, these giants, as you might call them?
They do say that one bigger than a tree was seen up away beyond the
North Moors not long back ... But this one was as big as an elm tree,
and walking – walking seven yards to a stride, if it was an inch.
This is all we have to go on, but it seems an Ent-like creature had been wandering around Eriador, and was seen by one of Sam's relatives in or near the northern borders of the Shire.