If you were able to post on Facebook, why didn't you have a GPS signal from which you could deliver your co-ordinates over the internet? (Just curious, not sure if I'm missing something)
As gp said, you could tell people exactly where you are, but it takes a long tome for anyone to be able to reach those coordinates by ship. If your power source dies and you drift in the interim, now it takes many ships and a coordinated effort hopefully involving aircraft for a chance that you’ll be found. The ocean is a very large place and a ship doesn’t exactly leave tracks.
different example, but same story, high altitude mountaineering, to quote Rob Hall's wife (famous guide who died 1996 on Everest): 'You might as well be on the moon.', she said during a live patch through to him via radio from her home in New Zealand, exchanging the last conversation.
Position is secondary, accessibility is primary concern