socionics and mbti functions are identical. (click on this link for reference to what i say below)
>the four last functions ought to be weak
this is just nonsense mbti talk that can not be projected onto a piece of paper that contains socionics theory. in socionics nobody is using the concept of "123 strong, weaker, weakest" or words "last function" or "series of function", because they know that a series of numbers only exists on paper, not in reality. and the order in which functions are mentioned in an article and how they are numbered is entirely irrelevant. what is relevant is their job.
first they list the ego block, the block of conscious attitudes, which has the dominant and the inferior function. i want to be clever (Ti) and i want to be free (Ne) could be the battle cry of LII ego.
then they list the super ego block, which is not a conscious personal attitude. as the word super ego already implies. it's about things that are perceived as given reality without ever being questioned. it's what mbti with it's 4 functions skips entirely because mbti is concerned only with what people will voice consciously or intentionally. but socionics is not designed to create superficial illusions like a mircosoft GUI. it reads more like a command line manual.
so then they list the resonance or shadow block, it's called super-ID, which lists the inferior function (the 5, or shadow function as carl jung calls it) and the tertiary function (the 6, which is the shadow of the auxiliary function). they list the inferior first, because resonance is as strong as what it is in resonance with, it's as important as the dominant function. they list the tertiary later, because it's resonating only with the less important attitude, the auxiliary attitude.
this is perfectly elegant. and identical to mbti.
now about socionics tests, i don't know a lot about how they work, but socionics is primarily a school of thought not a test.