Well.. Usually i always do this one many for the past years.
Building links about 50 within 2-4 weeks.
Recently I do link building about 50 links. After a week i was getting a good results immediatly, from #15 to # 7.
But this week, was my ranking is totally gone.. not even in top 100.. :(
I checked my GoogleWebmaster. It detected all the recent links that i build.
Seems like Google is watching artificial link building closely.