Owners of capital will stimulate the working class to buy more and more of expensive goods, houses and technology, pushing them to take more and more expensive credits, until their debt becomes unbearable. The unpaid debt will lead to bankruptcy of banks, which will have to be nationalised, and the State will have to take the road which will eventually lead to communism. Karl Marx, Das Kapital, 1867
Like the Marx quote, it's a bit too a propos. What "thousands" would King have been talking about? In which enemy's death was he supposed to be rejoicing?
A quick Google search turns up lots of tweets, all of them from today. Searching Martin Luther King Jr. quote pages for the word "enemy" does not turn up this quote, only things that probably wouldn't go over nearly so well, like "Love is the only force capable of transforming an enemy to a friend." I'm pretty sure that this quote, too, is fake.
What's fascinating is the speed of it. Someone made up a quote, attributed it to MLK, Jr., and disseminated it widely, all within 24 hours. Why? What do you get out of saying something pithy, and getting no credit for it?
Perhaps they only wanted to say this thing, and knew that no one would pay attention unless it came from someone else. Or, perhaps they are getting a gargantuan kick out of seeing people repeat their lie ad infinitum. Either way, it seems strange to me.
Update: Not malicious, but mangled. A personal thought was mashed up with an MLK quote through the power of the internet. My terrible, suspicious mind--and the fact that I only saw the fake part of the quote on Twitter--led me astray.
Meanwhile, over at PC World, Robert Strohmeyer illustrates how to check newly minted quotes using Google's advanced search features.