For those of you not caught up on Downton Abbey, I'll make this as unspoilerish as possible.
I've always thought "Sybil" was a great name, with its Greek roots and appearance in classical literature. Sybil is not in the top 1000 names in the US at present, though I have to ask if you can find fellow classical Greek names like Cassandra and Daphne on the list, why not?
For years there weren't too many Sybils out there in popular culture--you had the frightening Sybil Fawlty, and the book and movie Sybil about a girl with a personality disorder. Now with Downton Abbey there are two positive associations with the name. The show might not get the same ratings as a network hit, but the smart set watches it, and if they put the name out, others will follow.
On the other hand, the names of Sybil's sisters, "Mary" and "Edith," will continue their descent. Mary will stick around, but for decades it was #1 and now it's out of the top 100. I can't imagine it being top ten again, not in our lifetimes, anyway. And Edith? I wouldn't be surprised if it drops out of the top 1000 altogether. (Certainly the Downton Abbey character won't help.)