Let me disagree, Geostan. The choice of the preposition is sometimes a matter of taste and sometimes it depends on the context. In the present sentence, only
par is natural because the agent is
actively following the subject. When the agent is more "passive," both prepositions can be used but we prefer
de.
Il est constamment suivi par elle.

(not:
Il est constamment suivi d'elle.)
Ce wagon est suivi par un autre. (

) /
Ce wagon est suivi d'un autre. 