Windows 7 64-bit OS, but I have the same problem on my MacBook.
I have the Friends Feed enabled, but the sidebar only appears when I have the Spotify player maximized on my monitor. I have a two-monitor setup, but I don't want to lose an entire monitor just so that I can see what my friends are doing. If I un-maximize the player and have it in a regular windowed view, the Friends Feed goes away completely.
EDIT: I answered my own question. There is in fact a minimum size required for the feed to display; it's huge, though. My second monitor is 1600x900 and the player takes up almost all the screen space in order for the feed to be displayed.