I'd have to agree w/ both of you guys. yes, using more than one of those sites is borderline ridiculous, just because most have similar features/services... (I'll use facebook, MS & occasionally twitter if my boss is away on business)
BUT theyre also a fantastic way to network among friends, old school buddies, work contacts, friends of friends, etc... theyre breaking into a new social area, and i guess each site is trying to find their own niche, or area/service they can provide better than anyone else.