Bah, i'm trying to take revenge on people who had previously invaded my base but it seems I have to wait in line until someone else has finished first. This is slightly odd as 8 people have tried to invade me so far, some of which have multiple platforms but I can't seem to invade any of their FOBs. Any people had much luck invading foreign FOBs? I can't see any option to ally with people on my friends list which is a massive oversight on Konamis part, assuming i'm not missing something
For anyone who's considering taking a shot at invading someone else, I found abusing sniper rifles works, I found it hard to lose alert status when I was found trying to invade my own base and its only a matter of time until you get flanked by the apparently endless reinforcements that get sent against you. You don't need to worry too much about using suppressed weapons when you have cleared the platform you're on, just take your time and use the sonar frequently to make sure someone isn't going to sneak up on you and just snipe the men on the other platforms.