I've long had a theory that services that demand an upfront payment for an ongoing service will find the payment works against their growth goals and remove it.

For example, businesses that involve $$$ reusable delivery containers don't work well because they want to give first-time customers a discount not a surcharge. Whatsapp was ridiculously cheap - something like $1/year - but they still decided that worked against their network effects.

There are some markets where you pay upfront and on an ongoing basis, like buying a printer and ink or a games console and games - but they often subsidise the upfront cost with profit from the ongoing costs.

I'm not sure how well a $5-to-join social network would work for that reason. Sites like metafilter survive - but with five moderators and one coder, they're not going to be replacing twitter, facebook or HN any time soon.