Unfortunately there is still no dropbox support for FreeNASwith the now released Crashplan-Plugin the service of the same name can be used for a regular backup in the cloud (whether you want to entrust your data to a cloud company is another matter, an additional copy won't harm in any case).
Crash Plan can be installed via the FreeNAS plugin management. For the paid packages, Crashplan+ Unlimited is a good choice, where you get unlimited data volume in the cloud for 4 dollars a month - the limiting factor is then only the upload bandwidth, whereby after an initial full upload only the changes are transferred.
With the new plugin management it seems that finally some movement in the FreeNAS plugins is coming. Recently the Plex Media Server as FreeNAS Plugin appeared, further useful plugins will surely follow.
If you are thinking about setting up a NAS at home based on FreeNAS, I'd like to introduce my Shopping list recommended.
Me use crash plan now for a backup of all important files as well as all files that cannot be retrieved easily (holiday pictures). It is practical that you can restore the data of all PCs on which Crashplan is installed - in case of a crash you usually have no possibility to use the original PC anymore.