Rather than specifically answering each question and repeating what's already been said in parts, I'll perhaps make some more general comments.
Facebook:
As mentioned on the previous page, seeing who gets the most "likes" has it's advantages and disadvantages. Those with more contacts and friends who are able to share it around will inevitably win. So perhaps have two categories, one showing who won the 'like battle' the other some sort of more regular voting, with the judges just picking their favourite.
Competitions:
I often find that I'm uninspired by a lot of bases, and when there tends to be only one competition running at any one time, it means a long wait for the next base to be released. To help counteract this, it may be wise to have a few more competitions running, to give a greater choice. I really like Construct's idea actually, but I can see its drawbacks.
Another possible solution would be more themed comps like (i think) hugo mentioned, giving a free choice of base. But I guess the issue here is the advantage that picking the right base gives - it makes it hard to directly compare chops. Perhaps then a series of bases could be provided for each competition. Say for instance a drift themed comp, then provide a series of bases (maybe 3 to 5) that people can choose to modify. So a silvia, perhaps a V8 muscle car, some european car like a BMW and then some other more modern alternatives (GT86) - that sorta thing.
General forum activity:
No one wants to post or comment on a dead forum. Fact. So the key here is to create a culture of commenting with useful c+c. It can be demoralising when your work gets no replies, and it certainly won't inspire you to offer criticism on other works. In other words, what I'm trying to say is we all need to pull together to make things more active. It may just start with a few of us, but once momentum is gathered, it should be a kind of snowball effect. We need to make it feel more like a community again, more of a family! Perhaps a rep bar like so many forums have would help, with the ability to vote a post up or down, rather than just the chop itself.
But hmm. We'll have to really think any changes through I guess