Single moms are so 1998. The hot social-issue trend on TV these days is searching for biological parents. On ER, Dr. Kerry Weaver (Laura Innes) has begun a campaign to find her birth mother, as has freshman Julie Emrick (Amy Jo Johnson) on Felicity. Meanwhile, in the proposed Party of Five spin-off Time of Your Life, Jennifer Love Hewitt will shuffle off to New York to trace her real dad. And Opposite of Sex writer-director Don Roos just finished an NBC pilot, M.Y.O.B. (standing for Mind Your Own Business), which has a teen (Nicki Aycox) searching for her mother, only to find someone who may be her birth aunt (Elizabeth Perkins). What's with all the mommy (and daddy) tracking? ''It's a wonderful way to explore a character,'' says Felicity executive producer J.J. Abrams. ''It's a great search,and that's always good drama.''