I don't know about $5 to $10/month for Facebook. I would like to see a HN vote/survey on how much people would pay per month for the most used web properties if paying was the only way to see them. For me, the maximum monthly costs would be:

NYT. $15 Gmail $3 LinkedIn $2 Facebook $2

Not sure though about Facebook, HN, Reddit, etc. since they would be so much less valuable without large user bases.