The 5 minute rule gives us a simple planning rule of thumb for comparing storage technologies:
Given some access frequency, if an item is accessed more frequently than this break even time iops dominates the cost calculation. Likewise if an item is accessed more rarely, capacity cost dominates the calculation.
For current technology, roughly speaking, the break even time between RAM and SSD is 5 minutes, and about 6 hours between SSD and spun disk.
To cost optimize for a particular system you need to know the distribution of access times. For something like facebook the access times are heavily biased towards recently created items (ie, almost all users start walking the data graph from the top of the news feeds, and days old data is very unlikely to be touched).
This can make it difficult to get utility out of something like SSD that straddles a thin band in the middle of the storage hierarchy. Your dollars may be better spent on ram and disk in the proper proportions.