I am handling a site it have around 25,000,00 page and initial Google indexed around 20,000,00 pages till Jan 2013, after Jan 2013 Google De-indexed 18,50,000 page. i was surprised that what happened suddenly but not any clue come up. after 3 month i have apply for Google Reconsideration Request.but Google send report that it have not any manual penalty. Basicly my website have 3 Top TLD's .com, co.uk and co.jp this problem comes in co.uk site japan and .com site index properly. i already mentioned "hreflang="en-gb" on site. Kindly request to you please suggest me about it printsasia[dot]co[dot]uk
The index page will fluctuate and it is normal since this happens on every sites. But for your case, it seems like a huge drop which fall out of 'normal' fluctuation. Perhaps you have duplicated content on the 3 TLDs....
Did you check out your sitemap? maybe any error in the links? I had this ones, because of sitemap generator. As I saw if google sitemap found any broken link in the sitemap, it doesn't index more from this sitemap.