It probably depends on which site you are asking about and of course I’m talking about social media and cataloging sites, not real life. You might want to consider what a particular site means by friend before you accept all invitations or invite everyone you know.
Friends on Facebook was designed to be only people you knew in real life, but many people have used a normal profile instead of a fan page for their company, band or brand. Some games encourage you to friend people you know nothing about. I have friended many authors, but I wish they would create a fan page instead. How you select who to be your friends on facebook depends on how you use the privacy settings. If you leave everything public, it doesn’t matter because they can see everything either way. If you restrict everything to friends, including personal and contact information, you should be selective in your choice of friends. You can use lists to allow different levels of information and permissions to be granted to groups of friends. Examples might be only allowing family and trusted friends to see pictures of children or not allowing companies or other strangers to post on your wall.
LinkedIn has contacts instead of friends, but was designed to be people you knew and trusted, but recruiters and marketers will often try to connect with anyone. Twitter doesn’t have a friend designation, but you can only DM (direct message) people you follow if they also follow you. I want to clarify that anyone you follow can Direct Message you, even if they don’t follow you back.
Lately I’ve seen people from twitter who are new to Goodreads invite everyone to friend them on goodreads. On twitter everything is public and having lots of followers is a good thing. Lots of followers on goodreads is also good, but some people refuse to friend anyone who has more friends than books. On goodreads you can friend someone, follow them or fan them if they’re an author. You can become the fan of any author, not just the ones with a goodreads profile.