No, socionics was not meant to be the next step from MBTI. It was developed independently of MBTI. Hell, these MBTI-ish abbreviations weren't even used in the system originally.
And nope it doesn't make sense if you consider the descriptions. I'm not quite sure if Augusta (the founder of socionics) would agree that MBTI ISFP is socionics ISFj.
You misunderstand me.
It was independent, but they use ALL the same functions, terminologies, etc.
Just like Keirseyan temperaments.
Ugh fine I'll find you an article.
EDIT: oh wow I stand corrected. Introverts are ISFx, so you'd be right, an ISFP could very well be ISFp. Hmmm there ya go.
But, according to this link, ISFj is still MUCH more likely for ISFP than ISFp is. The reason being because an ISFp would be someone who is unsure about their Thinking/Feeling (even though Fi is the dominant function for the ISFP). So, an ISFp would feel more closely associated to the ISTP than the INFP (which doesn't seem right, but not out of the realm of possibility). If an ISFP finds that their Sensing and iNtuition are more in doubt than their Feeling and Thinking, THEN they'd be an ISFj (which should be most ISFPs, as I've already said).
Another example, me. I'm an MBTI INxJ. I don't know whether I'm an INFJ or INTJ, so I'm unsure about my two Judging functions but I'm CERTAIN about my Perceiving functions. Since I lean more towards INFJ, that would make me INFp. If I was CERTAIN that I used Fe-Ti as my auxiliary and tertiary functions, but wasn't sure if my primary-inferior functions were Ni-Se or Si-Ne, I would be an INFj.
So, in a sense, we're both right.
http://www.socionics.com/articles/howto.htm
I actually have an older cousin in his 30s that I struggle typing between ISFP and ISTP, BUT he's also EXTREMELY unhealthy (type of person that would punch someone that pisses him off which signifies undeveloped, inferior Fe, BUT also would get his feelings hurt rather too easily, which signifies possible unhealthy primary Fi).