I would say it depends on your definition of community.
A total of 58,857 (as of today) individual players have joined the Classic server at least once.
A total of 332 players have joined the regular Vanilla SMP Server from since we have been running the LogBlock plugin (March, 2012).
A total of 50 players have joined the Tekkit SMP server since we started running LogBlock (September, 2012)
These are just factual numbers though, it doesn't take numbers to make a community, it takes activity, and, well...
A total of 66 members registered on the forums have more than 10 posts. 7 of these are already banned, and are no longer part of our community, and another number aren't active any more.
This makes getting a percentage of the active community very difficult to collect, but, as a very very wide guestimate...
I would say:
<1% play on the Classic server at least once per week.
10% play on the Normal SMP server at least once per week.
89% play on the Tekkit SMP server at least once per week.
This surprises me, considering during the Tekkit server vote, people said they would play all servers equally, however: Normal SMP has been updated for clients, but not our server, meaning that it will be dead unless players really want to play on it, and the Classic server recently recovered from an issue relating to the Heartbeat system, meaning no one could connect to it, even if they wanted to.
TL;DR
There's no solid way to get an accurate percentage of active community members play on which server without first getting the figure of active community members.