You can send invitations to specific people via Facebook. I got one just a week or so ago. I would still say that my friend sent it to me "on" (or perhaps "through") FacebookWhat exactly did you do?
If you posted information about the reunion on Facebook, you weren't exactly, directly, informing any one individual.
Not me. For me, "on Facebook" is pretty much the same general idea as "on the phone." It's a communication medium for me, and I don't think I'm alone.if you told her about the reunion "by" Facebook, this is the medium you used to inform her of it; if you told her about the reunion "on" Facebook, this is where thte meeting is to take place (for me, anyway).
I'm not sure I understand your question, so allow me to clarify. For me "on Facebook" means "through the medium of Facebook." She (my friend) had told me about the open house in person, but the official announcement was an invitation sent via Facebook's messaging system. She told me "on Facebook," in other words.So for you, Kate, "on Facebook" could mean either of my suggestions?