I'm a big fan of social network maintenance-- I advocate being judicious in accepting requests and periodically weeding. Why? Mainly because it keeps things clean: loud signal, low background noise.
But in the process of deeply analyzing why this 'friend' decided to cease our relationship, I've been forced to inventory all the reasons others might engage in weeding, or pruning, as Peter Kim calls it.
1. Informational/ Strength of bond - Weeding friends is a method of filtering the potentially massive amounts of information coming in. It allows you to maintain manageable amounts of meaningful chunks of information. To be clear, as a measurement scientist, if I've unfriended you, this is why, with perhaps one exception.
2. Permissive/ Visibility - Cleansing connections allows you to control who has access to information about you (depending on both privacy settings and platform construction). Was there something about what she's doing now she didn't want me to see?
3. Identity Claim/ Reputation management - Who you're associated with says a lot about what kind of person you are: your career interests, opinions, beliefs, attitudes, etc. There are a few additional dimensions here too: the quantity of connections might indicate how popular and/or discerning you are, and the 'quality' of the relationships offers clues as to how influential or well-connected you are (and of course the inverse).
What else?
Others, like Clive Thompson, most recently have questioned the meaning of having hundreds of friends on FB, suggesting the technology and low barriers to online 'friendship' challenge Robin Dunbar's original work on network size. Perhaps this article in and of itself was what prompted her to remove her weakest ties (me?)...
But what's most bizarre is my lingering reaction... now 12 hours later, I'm still analyzing a list of potential reasons for being denied access to her doings. It kind of motivates me to want to re-connect. Is unfriending an effective strategy to elicit others to reaffirm the meaningfulness of your relationships? Ahhh reactance...