This “One Mighty Magazine Is Now Two” house ad promotes the split of Superboy and the Legion of Super-heroes comic title into two: New Adventures of Superboy #1 and Legion of Super-heroes #259. This ad appeared in Superman #343, cover dated January, 1980. Following their limited series in 1973, the Legion was folded into Superboy’s ongoing title which became Superboy and the Legion of Super-Heroes with issue #197. The Legion of Super-heroes took over the title with issue #259, and Superboy once again had his own title, albeit briefly as the New Adventures of Superboy only lasted until 1984.
Note the misspelling of the word “Electrifying” on the Superboy cover.