MFZ Flickr Group -- Do you like looking at pictures of cool mecha made of Lego? Yes? Then you should be a member. It's been around since the game was called Mechaton and has over 6,000 photos from the community. It has plenty of amazing constructions to gawk at and ideas to mine.
MFZ Facebook Group -- This is actual social media and is probably the quickest way to get whatever you want to talk about, you want to show off, or question you need answered in front of the most eyes. It also has an attention span measured in milliseconds so don't expect any long term discussions or results from here.
MFZ Google+ Group -- Similar to the FB page, but slower in terms of actual turnover of ideas. The comments are also a little more thought out and there's less "Lego spam".
Mobile Frame Hangar Forums -- For any matters needing long term discussion, this is the place you need to be. Comments and constructive criticism on your builds, a place to find other players, official background, long and short-form fan fiction, and a welcoming community are just some of the things you will find on the Hangar.
If you plan on being in to MFZ long term then you need to be a member on all four of these pages. It's that simple.