The episode Lost in Time, which aired exactly a week ago, was not only the most watched SpongeBob episode in the history of the show, but was also the most watched show in the last 5 years of Nickelodeon, with 8.6 million viewers. The details are all in a
Viacom press release. Some of the more interesting figures are that SpongeBob gets over 57.8 million viewers per month, and there have been 200+% increases in viewership since last year.
So, no need to fear. SpongeBob is stronger than ever, though the percent increases over last year are likely due to the fact that the first few months of last year were pretty slow on new releases.