As in forming actual, real-life social relationships (friendships, romantic relationships, etc.), virtual connections (with people or mediums) are formed based on trial and error. We stick with what (or who) we like, and let the rest go on their merry way.
I was a MySpace user. For approximately 47 days. I created my profile, added a few friends, uploaded some content, took a few surveys … and was over it. I was already “involved with” Facebook, and that’s where the people I identified with were hanging out. To me, MySpace was like the date you get asked out on, when you are already in a relationship, and that you are more than happy to politely turn down. 
I tried Twitter (am still trying!), but I don’t love it and don’t choose to spend much – if any – of my time with it. I have 37 followers, and follow about 30 “people” (most of which are actually brands, reporters and news outlets). I log on maybe once every six or seven days, or when I am alerted that I have a new follower – prompting me to remember that I have an account and update my status and read whatever is on my homepage (usually nothing but PSFK trend alerts, which I’ve read on their website already). I am not sure how much longer I’ll be a user, or if I will ever understand the social media world’s fascination with the medium, but I am determined to stay “in the game” on Twitter for now. For me, Twitter is the dating relationship that you want to work, but know is doomed to fail.
On the flipside, I was first turned on to Tumblr about a year ago, and it was love at first sight. Tumblr is a free microblogging site to which you can post text, photos, video, music, chat and more, all on a customized home page and within a community that makes me feel like a part of something. I spend time with it, use it and know it like the back of my hand. I have 171 (and counting!) followers and follow about 100 other users whose posts interest and inform me on a daily basis. Tumblr is like that one-in-a-million person that you want to spend all of your time with and prove that you are worthy of the same affection.
Why do we “fall for” one medium over the other? Why do we choose to spend our time with some mediums as opposed to others? In my opinion, it’s linked to the time and effort we choose to put into the “relationship.” For people who love to post photos … try Flickr, Shutterfly or Snapfish. For those who are interested in viewing and sharing video … there is YouTube, iFilm and Vimeo. If you are a blogger by trend or by trade … there’s Blogspot, WordPress, Tumblr and more.
Just like you pick your partner based on his or her traits and characteristics, so should you pick your social medium of choice. Make the commitment to spend time with your medium of choice, learning its ins and outs, its likes and dislikes … its personality, and your willingness to adapt … and look forward to a long, meaningful relationship as you become an expert at _____insert your social medium here_____.