User blog comment:Linathan/Unit Spotlight: Maestro Goddess Miku/@comment-26053138-20150904185244/@comment-26140304-20150907203015

520 is Wu Er Ling (五二零), which has a similar pronounciation as Wo Ai Ni (我爱你), which means "I love you". It's mainly used in SMS messages, similar to other abbreviations like "LOL".