topUsers.js

var osmium = require('osmium');
var users = {};

var reader = new osmium.Reader(process.argv[2], {
    'node': true,
    'way': true,
    'relation': true
});

var handler = new osmium.Handler();

handler.on('node', function(node) {
    users[node.user] = (users[node.user] || 0) + 1;
});

handler.on('way', function(way) {
    users[way.user] = (users[way.user] || 0) + 1;
});

handler.on('relation', function(relation) {
    users[relation.user] = (users[relation.user] || 0) + 1;
});

handler.on('done', function() {
    var sort = [];
    for (var user in users) {
        sort.push([user, users[user]]);
    }
    sort.sort(function(a,b) { return b[1] - a[1]; });
    console.log(sort);
});

osmium.apply(reader, handler);