Ask HN - why are more and more web apps requiring FB login?

I'm seeing more and more web apps requiring a Facebook account to log in instead of supporting OpenID - web apps which stand on their own and (other than the login) do not require Facebook to work. Is there a reason for this? Is it laziness, i.e. it's just easier to implement a FB login?

Or are there other reasons - are Facebook users are more valuable users to web app developers because they are inherently more likely to spread news about the service?