Jamieson, Fausset and Brown on Hosea 5:3

3. Ephraim--the tribe so called, as distinguished from "Israel" here, the other nine tribes. It was always foremost of the tribes of the northern kingdom. For four hundred years in early history, it, with Manasseh and Benjamin, its two dependent tribes, held the pre-eminence in the whole nation. Ephraim is here addressed as foremost in idolatry.

I how ... not hid from me--notwithstanding their supposed profound cunning (Ho 5:2; Re 2:2, 9, 13, 19).

now--"though I have been a rebuker of all them" (Ho 5:2) who commit such spiritual whoredoms, thou art now continuing in them.