here you have some fancy graphs!
(for mor significance ;-))

http://blockchain.info/en/charts
http://blockchain.info/charts/my-wallet-n-users?showDataPoints=false&show_header=true&daysAverageString=1×pan=all&scale=1&address=
http://blockchain.info/charts/n-transactions-excluding-popular?showDataPoints=false&show_header=true&daysAverageString=1×pan=all&scale=1&address=

about difficulty:
linear
http://www.bitcoinx.com/charts1/chart_large_lin.png
logarythmic
http://www.bitcoinx.com/charts1/chart_large_log.png


the one sure thing out of these graphs is that the community does grow slower than an exponential function.
 (its a polynominal form and not of the form a^bx)

this also means that it is not linear in logarythmic scale, why i dont believe in thefiniteidea's chart.