Technically, charges weren't dropped as there haven't been any charges (Swiss law works somewhat differently with regards to charging), but the original arrest warrant (for two allegations - rape and molestation) was withdrawn on 21st August 2010.
However, on 20th November 2010 an international arrest warrant was issued and was upheld by the Swedish appeals court on 20th November 2014. The prosecutors were criticised for not moving the investigation forward and then on 13th August 2015 they dropped the investigations into molestation and coercion as the time limit for those were due to expire (18th August). The rape investigation has until 2020 due to it being a more serious crime, so this story can carry on for a while yet.
Here's where I pulled this info from: http://www.bbc.co.uk/news/world-europe-11949341/