I was wondering the same thing. Seems there are very few populated servers anymore. As you said, hopefully things will pick up. The mod is great, hopefully word will spread and more people will start trying it. Plus as bugs are fixed and more content is added, it should attract more people.

It's like any other BF mod now a days.  Since the ranking system came out with BF2, it has killed the sustained popularity of all mods. 

Every single mod that has come out goes through this process.

1) Release = hype = popular
2) bugs and time = low numbers of players
3) version 2 or patch = a small return of players
4) map packs = a return of more players
5) core group is left at the end (low numbers)

PR is about the only mod that has sustained any kind of numbers over time.  The reason I think PR can do it is it appeals to the player that wants more of a "simulator" experience (like Americas Army) and that is the sole reason for it living on with any kind of numbers.

The things that will keep FS alive are:

More maps
Core group playing everyday
Frequent updates (news and versions) to BF community

I hope it lives on and grows in numbers, as I love this mod.