Ah! Here it is! It's from the Q&A section of www.socionics.com.

"'I don't understand the difference between introverted J/P types in socionics and MBTI. My MBTI type is INFJ, so you're saying that my socionics type is INFp? '

Not necessarily. If your type is obtained via either MBTI or Keirsey Temperament Sorter or any other type test (assuming that psychometric testing is 100% accurate), it should fully correspond with Socionics type. The "manufactured" MBTI type, which is not obtained via questionnaire but via so called "type dynamics" modelling, differs from Socionics type. How and why it is different you can find here."