It appears that Flock is suffering from the same memory leak problem like Firefox 1.5
I left my Flock running during lunch time with 5 tabs open one including gmail which would from time to time refreshes itself. After lunch Flock was using more than 500Mb then I had to stop it.
Found a way here to control the amount of cache uses by Firefox now having this tried out. I wonder if the same trick applies to Flock as well as the core is the same?
Alternatively you could sacrifice speed by disabling browser.sessionhistory.max_total_viewers! More details are here.
The official guideline for this is here.