Crista N. answered 04/24/20
Patient and Caring Spanish Tutor
This sentence is a complete sentence without the blank, because it just means "My friends send messages over instagram." Technically, that is a complete sentence, but it leads to the question, "Who do they send messages to?" or "A quien están mandando mensajes?" So, the blank needs an indirect object pronoun, answering the question, "to whom?" So, depending on what the sentence is actually trying to say, any of these would make sense:
Mis amigos me mandan mensajes por Instagram. (to me)
Mis amigos te mandan mensajes por Instagram. (to you)
Mis amigos le mandan mensajes por Instagram. (to her/him)
Mis amigos nos mandan mensajes por Instagram. (to us)
Mis amigos les mandan mensajes por Instagram. (to them)
Mis amigos se mandan mensajes por Instagram. (to each other)