I suspect that there is a closer correlation to one account = one person with Skype than Facebook since there is little social incentive to create multiple Skype accounts. In addition, the strength of relationships between Skype customers is more easily determined. Finally, Skype has the potential to scale well into MicroSoft's core B2B business model.
Facebook has 3x active over Skype - but as you mention, while Facebook are straight-up about their numbers, it is still hard to compare because of differing methodologies and different app types
I've wondered what impact Zynga has on artificially boosting Facebook's active user count.
Zynga's games provide an incentive to operate multiple accounts, and counts about 200m active users, so if only 5% of those users were each running multiple accounts (say 5 extra), that's around 50m dupe accounts. (I figure thats an over-estimation though)
Not to mention the other reasons people create dupe Facebook accounts
I suspect that there is a closer correlation to one account = one person with Skype than Facebook since there is little social incentive to create multiple Skype accounts. In addition, the strength of relationships between Skype customers is more easily determined. Finally, Skype has the potential to scale well into MicroSoft's core B2B business model.