Be the participant you wish to facilitate.
The most important job for a facilitator in an online learning environment is to facilitate learning — it is not to know everything. The best facilitators aren't the ones with expert-level domain knowledge, but rather the ones that listen, notice and ask good questions.
"Atta boy" comments, or compliments to others' posts are worthwhile and create a strong community for sharing.
Encourage those who are commenting and not sharing to participate, but don't force it. Some folks will hover around the edges to see if they want to join in. It's okay if they never do. It's also possible that they'll be some of your most active participants when they do.
One great aspect of sharing and shared purpose is that individuals progress or self-discover and are able to share that progression with the group.
What distinguishes wearing a "facilitator hat" or a "participant hat" is the motivation of the participant. As a facilitator, the goal is to engage the group and/or its individuals — to make them feel welcome, heard and connected. The facilitator is concerned with group dynamics and uses assignments and responses to prompt, connect and sometimes even provoke to move conversation forward. As a participant, the motivation is more self-directed, seeking knowledge and connections between oneself and others.
When a post doesn't seem relevant to the thread, or pushes on the perceived comfort zone of participants, a couple ways to handle that could be to redirect the focus of the conversation (e.g. a participant posted a photo of a bird shape she noticed on a cookie sheet and our facilitators redirected that into a conversation about bird identification) or suggest other ways for the participant(s) to contribute that may be more on-topic (e.g. a participant posted a cartoon drawing of a bird and our facilitators encouraged him to try other drawing activities that required similar, but more on-purpose approaches).
Some recommended technologies: Notational Velocity for taking notes about why and where you're going with individual facilitation moves, participants, and/or activities; ScreenFlow (Mac-only) for quickly recording a screencast to communicate instruction or an example; Google Drive for simultaneously creating content while designing the experience (this works as well sitting across a table from one another as it does virtually); Google Hangouts for meeting regularly with a team that is geographically dispersed; JotNot for creating crisp photos of sketches; Evernote/Skitch for annotating a screenshot; OmniGraph Sketcher (Mac-only) for quick and dirty data visualizations.
Even when you expect digital content (beyond words), as we did in Project FeederWatch: Sketch where most posts included photographs of drawings, you sometimes get a greater use of technology than you expected (e.g. Glecker's annotated PDF drawings).