Telling people is never easy but it is a massive step. And I don't see a problem telling them on facebook if that is how you feel comfortable doing so. It is completely up to you.

All I can suggest is tell them whatever you can. Be as honest as you can with them about what you are feeling and why you have distanced yourself. And if you can start by writing (or texting them) and letting them know that you have written them something that is important but may also be difficult to read and you want to make sure they are in the right spot for it. It allows them to not be as confronted by it if they don't already suspect anything and just think it is a regular 'catch up' message.

If possible I would suggest also telling them what you want them to do. Eg. 'I can talk about this if you want to call me or write back' or 'I'm not in the best spot right now but need you to know and will tell you more when I can'. A lot of people often want to help but are not sure what to do, and different people will want different things.

If you can find the links I would also send them some information on depression (links to websites etc) or suggest that they do a google search so they can find out information themselves. It then allows them to get more of an idea.

As I said it is never easy to tell someone but it does get easier. I hope everything goes well. And feel free to discard any of these ideas - they are things that have helped me in the past but everyone is different.