I don't think Twitter as defined here was ever a great markting platform. On the other hand, I don't think anyhting has changed to make Twitter less viable to those who get value.
Good marketing with Twitter has nothing to do with looking at the entire Twitter ocean, but with finding smaller streams of relevance. Search, targeting and, as many above say, engagement will always work.
Plus, Twitter is awesome for failing. Most failures (unless spectacular, frightening or hilarious) are forgotten quickly.
Twitter is dead? Long live Twitter