The Menacing weapon property says that you grant an extra +2 bonus to allies flanking a creature you are adjacent to. It never says that you have to threaten the creature, but perhaps that's just an oversight. It also never says that you actually need to attack the creature before the +2 increase becomes active like you'd need to attack with a weapon to activate the Defending property.
If you don't need to threaten then Tiny creatures such as familiars could grant the +2 bonus just by being in an adjacent square. That seems like it might be unexpected in terms of RAI. If you don't need to attack then a spellcaster holding a menacing dagger could grant allies a +2. If you do need to attack then double moving into position wouldn't benefit your allies until you got a chance to attack the next round, and even then I'd wonder just how long the bonus lasts - presumably until your next turn?
If merely threatening a foe with the weapon would be sufficient then what about a PC who carries a Menacing weapon in one hand while attacking with another weapon, perhaps a Monk with some Menacing nunchaku who flurries with unarmed strikes instead. I guess maybe Flurry of Blows could confuse that question, so how about a Sorcerer who attacks with Shocking Grasp while holding a Menacing dagger. Would he and a flanking ally get the +2 bonus or not? Does using one weapon without the TWF penalty prevent you from threatening with others?
I'd think that a halfling with a rapier who is adjacent to a giant would in fact be "threatening" the giant per the rules. The longspear does seem like sort of an odd case where the RAW that you need to be adjacent might not represent the RAI that you need to threaten the flanked enemy. Of course this is just my best guess.