Total fabrication, several incarnations of it over the decades:
snopes.com: Jane Fonda and American POWs in North Vietnam
Fonda did, in fact, bring mail from the USA to POW's in Hanoi, and she brought 241 letters from POW's back to their families in the USA. Fonda met only seven POW's, knew their names, which are now public record, and all seven of them deny ever "slipping her social security numbers". There was certainly no need, since they told her their names, and upon returning to the USA Fonda contacted some relatives of the POW's she'd met to relay information about the meeting. Those were the
only POW's she met in Hanoi.
I think what Fonda did was despicable, although I believe her motives were pure, if naive. She was very young at the time, and allowed herself and her hatred of the war to become a propaganda tool, something she now regrets. I personally hate what she did. We've all done things that in retrospect we wish to hell we hadn't. But this mass of vicious lies being propagated over decades is frankly more despicable than anything a foolishly naive young woman did in her desperate zeal to end a war that a huge hunk of this nation was completely against in the first place.
Now let the sputtering of "LIES!!! LIES!!!" begin.