I don't go on Myspace often, if at all. But I will typically approve a friends request if the person seems vaguely familiar to me.
Facebook is generally where I friend all the people I'm acquainted with on a fairly extensive level--meaning we've spent time together at some point in time.
And livejournal. My livejournal friends list is reserved for any of several categories of people: a) people whom I do not know, but share multiple common interests with, and who're interested in reading about my life (and vise versa). These people have no connection to me other than livejournal, typically. b) people I know pretty well, and trust implicitly after years of friendship, and c) people whom I've met, immediately clicked with, and want to become closer to.
I value all of the above groups pretty highly, and the common theme among all of them is trust. With the first group, I can trust them not to judge me or use what I write against me because they have no idea who I am other than what I type. With the second, I've known them so long that I understand their character pretty well, and with the third, I take a risk and let them in in the hopes of fostering a better friendship.
If I choose not to friend you on livejournal, or if I de-friend you at some point in time, the issue at hand was probably related to trust. If you're one of the people I thought I knew well after an extensive friendship, but then you make sniping comments about me under your breath, or frequently filter me out of your journal entries, you'll probably get defriended. It doesn't mean I don't like you, or don't want to spend time with you...it means I don't trust you to be inside my head, reading about my thoughts and dreams and desires and whatever else I choose to put on livejournal. If you are someone I'm acquainted with, and I don't friend you, it isn't because I harbor a secret hatred for you, just that I don't feel like I know you well enough to let you know me in this way. Yes, even if we have the same circle of friends and frequently see each other--that doesn't equate to closeness.
I don't think that any of the above is confusing or unfair. This is a forum for me to share things about myself that I don't normally feel comfortable sharing in other places.
In other words, this is my way of announcing that I'm doing a friends cut.
If you've been defriended, and feel like I was mistaken about you, I'm all about second chances.