FDN was formed through an alliance encouraged by the US between the Somoza Guard and MILPAS - a Sandanista breakaway group that gathered its support among peasants from the mountain region. The latter group were far more numerous.
Oh I forgot that faction led by a certain Tigrillo. I don't know in which sources are you sustaining your claim, but some authors think that the importance of the MILPAs was very very relative. In other words, more important in symbolic and propagandistic terms than in military strength or actual support among peasantry:
From
Peasants in Arms by Lynn Horton (Google Books):
In any case, do you think the Contra struggle had legitimacy as a liberation movement, or was it only a tool to fight a national liberation movement which coincidentally dethroned a bloodthirsty dictator and loyal friend of the USA called Somoza? Do you think that terrorists are "good guys", as long as they are "our guys"?