Do ants also use the Earth magnetic field?
Hard to say, but for at least one ant species (see below) it seems to be an orientation cue for (relatively short-distance) migration (even if you're only going a few meters, it can be important to walk in a straight line!). At present it seems we know something about the proximal mechanisms of magnetoreception, and a little bit about the observed behavioural patterns, but don't have much more than informed speculation about the ecological/evolutionary advantages of this form of spatial orientation.

Searching "ant magnetotaxis" gets to this article on magnetotaxis in the migratory ant Pachycondyla marginata:


  The influence of geomagnetic field changes on the behavior of ants has been studied in the species Solenopsis invicta (Anderson and Vander Meer, 1993), Formica rufa (Çamlitepe and Stradling, 1995), and Oecophylla smaragdina (Jander and Jander, 1998) ... Another migratory insect species is the ant Pachycondyla marginata, which has very peculiar habits: a diet of living termites and migration in a preferential direction (Leal and Oliveira, 1995). These habits make such a migratory ant very attractive for the study of biomineralized magnetic materials.


From Leal and Oliveira 1995 ref above:


  Migration by P. marginata colonies lasted over 2 days and covered distances of 
  2-97 m (n=48). 


Selections from Acosta-Avalos et al. 2001 Naturwissenschaften:


  abstract: Migrations during the dry/cold season are significantly oriented 13 degrees with the magnetic North-South axis, while rainy/hot migrations do not exhibit a preferred direction. This result is discussed considering the hypothesis that P. marginata ants may use the geomagnetic field as an orientation cue for migrations in the dry/cold season. 
  
  Given that colonies resume nest relocation late
  in the afternoon of the second day, most of the migratory
  process takes place under darkness conditions.
  
  On  the  other  hand,  nest  relocations  in  the  dry/cold
  season can be considered as typical migrations.
  
  The  use  of  geomagnetic  cues  is  more  commonly  associated with darkness conditions, as reported for mole rats
  (Marhold et al. 1997) and also proposed for bumblebees
  (Chittka  et  al.  1999).  As
  P.  marginata
  ants  start  the
  migratory  process  in  the  afternoon,  they  can  use  any  of
  those  cues  in  their  orientation  during  the  migratory
  journey. However, it is intriguing that they prefer an axis
  around the North–South axis to migrate. The only possible
  cue  to  give  this  information  under  all  conditions  is  the
  one  provided  by  the  geomagnetic  field.