Gossip Girl 8 p.m., Channel 56

I was a huge “Gossip Girl” fan until, about three years in, I wasn’t. Nighttime soaps often get tedious as the plot twists become increasingly absurd, desperate, and repetitive. “Revenge” is beginning to show symptoms of that syndrome. But as “Gossip Girl” ends its run on Monday night, it’s time to celebrate one of the first series to capitalize on social media and make it an essential and natural part of its story line. The two-hour finale will include a big reveal: the identity of Gossip Girl. My guess: Keyser Söze. Pictured: Leighton Meester (left) and Blake Lively.