What are the downsides to using Facebook as a communication tool? I have: a fake email, a fake name, no mobile number attached and I don't post statuses.

Facebook groups and messages however are just so incredibly convenient and ubiquitous that you won't be able to convince entirely separate groups of people to leave and use a service you mandate - unless you are friends with solely technical people.

Could I be a member of the groups and organizations I am without being in the Facebook groups? Sure - but it would inconvenience everyone, if there's a discussion in a group, and someone tags me to get my opinion on something, I can fire back a reply. They don't have to call or email me separately.