It's been a few years now since Dropbox ceased support for PowerPC and although there was a community workaround that worked for a while, that too fell by the wayside with ongoing developments at Dropbox.
Folder syncing has never been a specific requirement for myself but having convenient access to a central repository across various machines is always useful - for this I use the WebDAV protocol supported by various online storage providers either via Finder in OSX or with the excellent WebDAV client, Goliath (which I personally find delivers faster file transfers.)
So, after signing up to an online storage provider - one that includes WebDAV access - in OSX Finder go to Go/Connect to Server (or press CMD+k) and enter the providers WebDAV URL and press Connect. Enter your username and password in the next dialogue and tick box to remember password.
All being well, the remote server will be mounted onto the Desktop and be available to all standard file operations just like any attached storage. For convenience, I make an alias of the mounted drive, move it to Documents and drag to the dock to provide instant access in future.
The procedure is similar when using Goliath - making a new connection, enter URL, username and password and once connected, go File/Save Connection As to save a WebDAV bookmark that can be launched from the dock if desired.
Of course, the advantage of Goliath is that it's available for OS9 too - it's quite remarkable at this late stage to still have the convenience of online connected storage on such an old OS.
In addition, all these online storage providers have a web interface accessible from anywhere and usually have purpose built mobile apps for Android and iOS - in addition, there's always mobile WebDAV clients too - even my beleaguered Windows Phone 10 powered Lumia 930 can get in on the action with it's WebDAV equipped Total Commander file manager!
WebDAV access is a refreshing change in these twilight days of PowerPC - an online service that spans legacy systems and doesn't tax the CPU in any way.
For those wishing for a more Dropbox like folder syncing facility, with a compatible provider rsync can be utilised for effective backups but for now WebDAV suits me fine.