You are lucky in terms of timing since a lot of people have been posting growth rates for facebook, twitter and Google+ on Google+.
Check the linkedin growth curve post and a slightly dated Steven White blog post on 5 year growth rates for the big 5 With respect to your first question, if you were to model and simulate your business what factors would drive user growth?
For example if you run a job site the numbers of jobs posted and resumes received would be linked to economic growth and cycle and the unemployment rate at a high level. If you were to run the same site but focused on financial services in the Far East, you would need to get a lot more specific.
Figure out what the core driver is for your business and sector that would drive the underlying population and then what portion of that growth can your business capture. Also think about the process that defines your share of the market.