libappindicator1 may be the answer but in many cases it isn't.
The following has been submitted to www.dropbox.com as a bug but linux users don't appear to be a high priority.
dropbox's preferences, if you check
Start Dropbox on system startup, the daemon will start and
dropbox will be working in the background, but the icon will not appear. This is apparently because the
dropbox autostart config file
~/.config/autostart/dropbox.desktop insists on setting a startup delay of zero but displaying the icon requires some other process to be running already... and in many cases it is not, especially when a number of other startup applications are starting up / running.
In Ubuntu's System Settings/Startup Applications, if you set the startup delay for
dropbox to 10 seconds or more, the process works properly and the icon appears... once. Then
dropbox resets its startup file
~/.config/autostart/dropbox.desktop back to a delay of zero and the problem returns.
Workaround: uncheck the
Start Dropbox on system startup, write a short script that starts
dropbox, add the script to Startup Applications and set the delay to 10 seconds or more as needed by the Ubuntu distro and by other startup settings.
dropbox should stop rewriting the delay in the autostart file
~/.config/autostart/dropbox.desktop or make the delay a preference.
dropbox stop #because the actual daemon usually IS running
Save it as something like
fix-dropbox-icon.sh. Remember to make it executable in the permissions.__
Unrelated point of interest. After doing this I reviewed my Startup Applications list and changed everything that I don't consider immediately vital for a startup delay of 30 seconds or more. Now my Ubuntu startup is MUCH faster. Of course I have a lot of stuff there. Your results may differ.