Familypedia used to have an email list. Wikia discontinued such lists several years ago.

In late May 2010 we had "22 Non-digested Members of Genealogy-l" and "7 Digested Members of Genealogy-l" (!).

Now there is no sure way to send a message to all active members. A blog post is probably one of the better ways, but the forums may be appropriate, depending on the subject-matter. Both can be accessed via the "FORUMS, BLOGS" menu at the top of most pages. In either case, try to make your page name and edit summary relate to the content, not just something like "My idea" or "Genealogy news".