Great question... one I have been thinking about a lot lately.
I have a lot of "friends" on Facebook whom I haven't had a conversation with in 20+ years, and yet it plays a much more important role in my life than I should let it and I often need to very conscientiously unplug from it for a while sometimes. On the other hand, it allows me to remain close to family members in many different cities & states. I am very careful to not post something too provocative or controversial - that's not really who I am anyway.
I have friends - one in particular - who is a complete introvert in real life but is someone else entirely online and posts stuff she would never say in conversation.
I think a lot of people, particularly younger folks, frequently try on different roles online and reinvent themselves. I am SO THANKFUL social media wasn't around when I was a teenager... :)