There are some down right incorrect answers here.
From a sociological standpoint, yes, everyone who encounters conversation on a daily basis can be brainwashed. Using a very liberal definition of the term, it simply means that you take to be correct something that someone else has said.
If you want to get deeper into the subject, the brainwashing done by cults is an excellent topic to research and can show you a darker side of the word. Then, from the research you do there, you can see just what kind of person can be "brainwashed" in the sense that they blindly agree to a new set of social rules.
These kinds of people, those in a cult, are more susceptible to a cult leaders words for a plethora of reasons. Some of those are:
-Little to no social environment
-Little to no social fabric/support system
-Some kind of mental instability
-Low self esteem
Hope this helps, it's a very interesting topic
Minor in Sociology