not sure, hopefully we get an gamexplain video or someone shows us. I don't have a chance today to check it out myself. I assume it shares the replays to Miiverse, but perhaps social too?
good question. i would assume the sharing stuff might be in the 3ds version, mewtwo support. not sure about the mii fighter costumes and the 8 players stages are not supported obviously.
3DS has had sharing since 1.0.5. You can upload replays/snapshots/mii fighters to an online database, which people can look through and download from. They're deleted after a set amount of time (30 days iirc). All DLC is shared between versions (and you get a discount for buying it for both versions).