What different series are or have there been designed primarly for girls?
As you've seen, the LEGO Friends sets are the latest theme aimed primarily at girls, however there are a number of current, and even more historical themes.

Other themes have included:


The Powerpuff Girls (2018 - ) - Uses Minifigs with custom heads, based on the popular Powerpuff Girls license

Elves (2015 - 2018) - Uses Minidolls similar to the Friends sets and continues the pastel colour themes (as of December 2018 all sets listed in the LEGO store are "Retiring Soon").

"Disney Princesses" now just Disney (2014 - ) - The sets based around the Disney Princess licenses use Minidolls as well, with a similar colour palette to Elves and Friends.

Belville (1994 - 2008) - used many standard LEGO bricks, but the figures were much larger than Minifigures.

Clikits (2003 - 2006) - a range of jewellery that can be used with LEGO System elements.

Disney Princess (2012) - a sub-theme within DUPLO based on the popular Disney Princess range, featuring some of your favourite Disney characters.

Friends (2012) - based around the LEGO System elements, however the Mini-dolls are slightly larger than Minifigures.

Homemaker (1971 - 1974) - a very early theme based on LEGO system elements but before the minifigure

Paradisa (1992 - 1997) - a sub-theme of Town, using standard minifigs.

Scala (1979 - 1981, 1997 - 2001) - the original theme used LEGO System elements and was jewellery based, but the more recent theme bears very little resemblance to LEGO System elements, although some plates and other bricks are occasionally used.
 


Set/Box images from Brickset