In a riff on "If you build it, they will come", many online services are seeing their paid subscriptions increase when they add storage AND make it easy to use. The real key is making it easy to use.
Google sells extra space to use with Picasa, and Google Docs. Online networks like Ning and Multiply offer lots for free, then you can remove ads and store lots more content if you pay a small subscription fee.
Multiply.com recently released a mass upload utility which has resulted in lots of additional uploads and many more subscribers thumbnail ID images have suddenly started showing the 'P' icon which designates premium subscribers.
Many social networks expected to monetize in other ways, but any revenue will do. So what if that big social network is just a front for a nice online storage business.
The next wave in storage is online, and the shift is happening now.