I don't remember MFAN, but there were some memorable ones.
the time my friend had a PS3 (when they were new) and I turned up for like a week solid to play Metal Gear 2, and the time he went to bed at 3am, and I just sat there playing Grand Theft Auto straight through from about 11pm to 12 the next day, didn't even get up to pee. I only knew it was morning when he got up to go to work, and I realised the sun was up outside.
The Laser Quest (like laser tag) all nighter - I used to hang around there with my friends pretty much daily, and the first all nighter started the very night I got back from holiday in Portugal, straight from the airport. All the Arcade machines were set to free play (Xexex, some Neo Geo fighter, some other stuff), and we were playing variations on standard like Royal Rumble and AVP in the arena etc. It was pretty loopy.
And then (soppy story) I remember me and one of my girlfriends just went for a walk one night, and just kept walking around the city getting into conversations and doing stuff until the sun came up.