The working paper site RePEc has just launched its own directory of economists who tweet. This currently lists over 150 who fulfil the criteria of inclusion given on the website.
If you have not visited RePEc recently, it now indexes over 2 million economics, finance working papers many leading to abstracts and full text. The site also links to EconAcademics.org which aggregates postings from key economics blogs and is hosted by the Research Division of the Federal Reserve Bank of St. Louis.
There is a link to RePEc from the Economics subject page: http://www2.le.ac.uk/library/find/subjects/economics