I have found two things that specifically help Facebook groups grow.
First is setting an example. I have found that once the community gets the idea of what content the group is for, they will start posting it. If the group name and description don't make this clear enough, examples can help - a lot. The usual seeding of posts to appear each day is one way. Another is to have a theme for each day of the week. When the community posts something give them a lot of love in the form of replies and comments.
Second is starting conversations (and stimulating). Nothing cements a community like a few interesting conversations. Posting daily questions - especially very open-ended questions can get the conversations going.
Either way, look at the stats for the group. Over time, a few members will stand out as top contributors. Build a relationship with these people. Some will be your future moderators, others will just be top contributors. Help them to have a sense of ownership in the group.
As a bonus idea, it can help to make a group look better engaged by finding older posts by other people and commenting on them (again). Not only does that cause the previous likers and commenters (and OP) to come back to it, but the post is returned to the top of the group.