The user should be able to completely choose the information that he or she is willing to provide when going online. Public domain artworks should not be censored by private companies. In fact, national institutions should not turn to private solutions because it is easier or more efficient. And although I am not an active member of the libre software community, I do share some of their core principles. For these reasons and then some, I am not very active on commercial social networks.
That being said, I once had an account on LinkedIn™ for professional reasons. For personal ones, I had one on Facebook™. Sometimes, I experiment with Twitter™.
As you can guess, I have a love-hate relationship with social networks. They consolidated some friendships that, because of distance or time elapsed, were not necessarily part of my daily life. I'm thinking about old school acquaintances found on Facebook™ as an example. However, as an art historian, I first felt uncomfortable when L'Origine du monde by Gustave Courbet was censored. I decided to close my account when targeted advertising appeared in the middle of a conversation with a loved one. I turned the computer off, picked up the phone and talked with the person concerned. Immediately after hanging up, I closed my account. Since then, I've had no regrets. Just spend some time on the websites of Richard Stallman, the Free Software Foundation or the Columbia Journalism Review to be convinced of the usefulness of such a gesture.
Regarding LinkedIn™, the situation is more ambiguous professionally since this is a work resource. Indeed, many employers expect to find your profile on this site to get an overview of your career. Why exclude oneself? In this context, I tend to open an account temporarily, if required, for the time of the interview. Then, if there is an opening, I try to explain to those involved why I do not like this tool.
Surely, Academia.edu is better for academics, right? Well, when even Forbeswrites that scholars should delete their account, I feel mainstream.
Twitter™ is the most ambiguous social network for me. It is the least intrusive in terms of identification of persons or use of private software. Again, I refer you to the sites of Richard Stallman or the Free Software Foundation for more information. The fact that the FSF is on this social network raises legitimate concerns for some members of the libre software community. In the end, for me, it has the same flaws as Facebook™ and the others (Instagram™, Snapchat™, etc.) In spite of these concerns, once in a while, I open and close Twitter™ accounts (former usernames: @mgauthier1909, @circa1909, @Mrc_Gthr). Sometimes, I maintain @MrcGthr.
In this context, do not be surprised to see me appear or disappear from your networks. It's not you, it's me. And, if you feel like it, you might want to learn about alternatives that are more libre like diaspora* or Mastodon.