Find and add a couple more players. We've got six and it's a little thin. I don't think I could deal with more than 10, but a few more would be good.
Get everyone involved in the main plot so they feel that they're an important part of it. We started an invasion of Earth storyline and hadn't really had a plot that spanned the whole game before. I think that'll help.
Do more chatting with other people in the community. I've already started on sites like this and on reddit and it's going well. Mostly everyone is very friendly.
Maybe start a social media account for the game, like X (Twitter) where I can post a "post of the week" or something.