From my point of view,
Flock = Mozilla Code base + Firefox Extensions + Some UI Integrated.
I reason why i joined Flock community because i am very much interested in R&D kind of things. Since Flock Development started just now, i thought it could be a good journey in my techno life. Flock is some what built using Firefox 1.0.5 version of source. check out Flock.com for more information.