Lots of reasons (had to research them for a dating site I'm doing at work)
Terms and conditions changes
Most (as in the vast majority) of 'members' are not active
There will be a significant cost in vetting these members
Many users will have multiple accounts for 'creeping'
Most users only sign up to look
So of say 10 million members, 1 in 10 will be active subscribers, then only 1 in 10 of those will be using the service at any one time, makes the numbers look good though using total members

Edited as I fail at list view

_________________TwitterCharlie Brooker:
Macs are glorified Fisher-Price activity centres for adults; computers for scaredy cats too nervous to learn how proper computers work; computers for people who earnestly believe in feng shui.