I forgot to check last Friday, but as of today: in 2015, 2035 articles using MTurk (or mentioning MTurk) have been published according to Google Scholar.
At the end of February: 705 studies
At the end of March: 1070 studies
At the end of May: 2530 studies
Wow, April was a busy month! MTurk is on track to be in about 7000 studies this year.