Filters change looks
Instagram’s filters change people’s looks greatly. This can give a wrong impression about a person as they might seem much more attractive than they really are.
False attachment to people
According to psychology, a person can get attached to people posting pretty pictures even if it doesn’t really look like them in real life. In such a case, even if a person knows how a person looks in real life he can still get attached to his pictures on the unconscious level.
People faking happiness
Many people try to post pictures to show that they are living a happy life. This kind of behavior can result in getting a false impression about others. This could lead to sadness or even depression.
People faking their life style
Many people try to broadcast a certain life style through the Instagram pictures they post. Those people can give the false impression that they are living a certain life style, whereas they could be living a totally different one. (See Why do some people create fake social network profiles?)
Fake & shallow relationships
Because Instagram has much less communication options than Facebook and is generally more suited for passive viewing, a person might think that he has developed a good relationship with others using it, whereas it could be just an illusion.
Peer pressure leads to more fake posts
Because Instagram users are usually subject to heavy peer pressure, they might be forced to fake a life style that they are not living or to modify their own pictures to make them look better. (See How peer pressure may influence an individual?)
People showing the best side only
Under the effect of the tough competition people face on Instagram they get forced to only show their best side unlike on Facebook where people usually share both the positive and the negative.
You can hardly know people’s personality
Unlike Facebook where a person’s personality can be revealed through his posts, words and thoughts, Instagram can hardly allow a person to really get to know someone as it’s focused on pictures.