What we typically consider to be noise in our social stream is easy to spot and eliminate. If someone posts a steady barrage of messages that are outside our areas of interest, we can easily remove them from our friend/followed/circles list. Even if we slack on housekeeping chores, the incremental negative impact of each uninteresting post is negligible. We can scan through dozens of foursquare check-ins, links to cute cat pictures and Klout score announcements in a few seconds. Little harm done.
The real threat to our time and attention isn't noise, but the the posts we usual think of as signal. The ones that engage our interest, capture our attention, but don't contribute lasting value to our lives.