Pluto in Libra rules my 11th house and conjuncts Saturn in Scorpio and I can relate with what you're saying, I tend to like having just one close friend at a time, I'm not possessive though, my friends can have as many friends as they want and I tend to become friends with their friends. Therefore once I got married my spouse became that friend that I focus on and my career is the other thing I focus on and I have Pluto in the 10th. Since I am a counselor my line of work is innately intimate and social so I don't feel motivated to socialize so much outside of work but like you I feel like I should put in more of an effort in making more friends and building up my social life more.
Something else I have noticed about Pluto ruling my 11th is that friends can turn into lovers or lovers turn into friends. I also tend to keep my some of my friends secret.
I also feel pluto in the 11th or ruling the 11th can attract friends who need to be in control and want you to depend on them or who depend on you so eventually you realize these are both bad options.
I do enjoy friends with benefits 😂
There are a few friends I feel there was real intimacy between us and a strong emotional connection and it was not sexual, so nice when that happens and that is ultimately what Pluto in the 11th wants real intimacy, a Strong emotional connection but between two strong healthy adults who don't need each other but feel nourished by one another but without demands or expectations.