The parallels between Easter Island and the whole modern world are chillingly obvious. Thanks to globalization, international trade, jet planes, and the Internet, all countries on Earth today share resources and affect each other, just as did Easter’s dozen clans. Polynesian Easter Island was as isolated in the Pacific Ocean as Earth is today in space. When the Easter Islanders got into difficulties, there was nowhere to which they could flee, nor to which they could turn for help, nor shall we modern Earthlings have recourse elsewhere if our troubles increase. Those are the reasons why people see the collapse of Easter Island society as a metaphor, a worst-case scenario, for what may lie ahead of us in our own future.

Jared Diamond (b. 1937) American geographer, historian, ornithologist, author

Collapse: How Societies Choose to Fail or Succeed, Part 2, ch. 2 (2005)
    (Source)

This is the actual full text from Diamond's book. It is almost universally paraphrased (including the bracketed inclusion) as:

The metaphor is so obvious. Easter Island isolated in the Pacific Ocean -- once the island got into trouble, there was no way they could get free. There was no other people from whom they could get help. In the same way that we on Planet Earth, if we ruin our own [world], we won't be able to get help.

I speculate that this pared-down phrasing was used by Diamond during a speech or seminar about the subject, or an interview about the book, and was then mistakenly identified (and copied) as a quote from the book. For example, at the ASA, CSSA, and SSSA Annual Meetings, Long Beach, California (2010), for example, Diamond is quoted with this near match:

The metaphor is so obvious. Easter Island is isolated in the Pacific Ocean; once the Easter Islanders got into trouble, there was nowhere that they could flee. Just as if, today, we on planet Earth mess up our island planet, there is no other galaxy that we’re going to be able to float off to.