I agree with Tomsen- wouldn't want it to spam anyone. A simple "max update interval" setting of 1 day would be easy I think, or twowheel's method of no stream updates with progress. Although between my brother in law and myself, I like to see how he's going with books I'm interested in reading so I can bug him about what he thinks so far.
HarryT - that is poor form, I'm sorry to hear it. I will say this- it's mostly just a glorified website bookmark at this point. Without true integration it doesn't get us too far yet. However, the look-forward features etc., do appear nice- although I have just started using the new os.