For 4.1 years, from Saturday 9 July 2011 to Sunday 16 August 2015: 207,380 tracks played in total. Currently showing: 229.417 tracks. Meaning I lost over 20.000 scrobbles. People have been reporting this issue since yesterday (http://www.last.fm/forum/21713/_/2277540/3) and it appears that nearly everyone is affected by this issue. It says that on the BETA version, everything seems to be in order, however. I cannot acces the BETA currently, so I am not sure if my profile looks alright on there.
I lost all the scrobbles for ザアザア yesterday, and this band isn't scrobbling currently, either. I don't exactly know how many artists I am missing, but I reckon it's a bunch (grand total is currently showing over a thousand, so I lost track on how many artists my library holds). Lost over 5000 scrobbles on KAMIJO, and a lot more on other artists as well.
edit: I can confirm that only profiles on the current version of Last.fm are affected. I just switched to BETA and everything is still there. I don't know exactly what causes this as we've been given no answer as of yet, but I hope they manage to fix it.