NBC's string of Thursday editions of Saturday Night Live's Weekend Update got off to a competitive start last night. At 9 o'clock, the half-hour special averaged a 1.7 rating among adults 18-49. That put it behind only CBS' typically dominant Big Brother in the key demographic. Weekend Update, which also jumped 42 percent from its reality lead-in, averaged 6.5 million viewers for the night. This is just a four-week run for Weekend Update's Colin Jost and Michael Che. The show will wrap when SNL gets back into full production.