A) Thousands (meaning as little as what 3,000) worldwide - Yes? Significant numbers compared with a) the worlds population or b) even the population of JWs? NO.
B) They probably aren't any LESS effective since door to door is the biggest waste of time there could possibly be. I'll bet more literature is placed via the carts than on a typical weekday morning where no one is home. But is that an effective means of gaining converts, uh NOPE
C) I think its really the only thing they care about.
The real issue is that door to door work 1) keeps members busy, 2) allows for a shared miserable experience which helps galvanize an "us against the world" mentality and a persecution complex (if this wasn't Gods work than Satan wouldn't be trying so hard to stop us by making it difficult). and 3) provides a way to measure "spirituality" to determine if men qualify for leadership positions. Although you have some other touchstone (contributions!). There's no way that any JW thinks door to door is effective.
That being said, I don't see them dropping anytime in my lifetime. It's one of the fundamental parts of being a JW and about the only thing the general population knows about them. It's a signature trait.