This Lincoln Park spot has just enough space for the essentials--a bar, a sprinkling of cocktail tables and a pool table with maneuvering room around it. Throw in a dash of mood-setting decor--candlelight, polished woodwork and a veritable rainbow of lava lamps--and you have a snug, sophisticated lounge. But Frank's, which pays tribute to Ol' Blue Eyes on the walls and in the jukebox, has a split personality. Since it's one of the few joints in the area with a 4 a.m. license, it serves as a magnet for twentysomethings looking for a place to party after the North Side's 2 a.m. bars close. There's usually a line outside late into the night, while inside it turns into a mosh pit.