Yeah. That's not really a business model though, more of a pipe dream. You cannot support a service with one-time "very small" optional charges. Unless, of course, you start to twist users arms, in which case these charges stop being optional.
Optional charges seem to work pretty well for Wikipedia. Although who knows how well that will transfer to a social network.
$5 will buy a full month of server time on a modern cloud host. I imagine that if they can extract that much out of a user, they can make it go a very long way. This is especially true if the optional charges include things that consume large amounts of disk or bandwidth (video/pictures and associated long-term retention).
$5 a month out of each and every user? Good luck with that - it would put them over Google and MASSIVELY beyond Twitter and Facebook (both in single figures annually).
Exactly. Tumblr couldn't put money together with the themes people were buying, even though that was pretty lively.