Actually the Free Masons are awful at keeping secrets.
Any secret organization
worth its salt wouldn't have hundreds of conspiracy theories about it. Heck,
we'd probably not even know they existed.
Knowing it exists and that there are secrets is all that is required
for people to come up with conspiracy theories, they don't need to
know any of the secrets - there is plenty of smoke without fire.
Ideally, we wouldn't know it exists, but it's been around for so long
that someone was sure to notice sooner or later.