Another strategy is to use one-db-per-user, but add a server-side process to occasionally dump user databases into some global database. E.g. here is a simple Node app:

var Promise = require('bluebird');
var request = Promise.promisifyAll(require('request'));
function syncEverythingToGlobalDB() {
  return request.getAsync('http:https://localhost:5984/_all_dbs').then(function (dbNames) {
    // find your user databases, however you named them
    var userDBs = dbNames.filter(function (x) { return x.indexOf('userdb') === 0; });
    return Promise.all(userDBs.map(function (userDB) {
        return request.postAsync({
          url: 'http:https://localhost:5984/_replicate',
          json: {source: 'http:https://localhost:5984/' + userDB, target: 'http:https://localhost:5984/globaldb'}
       });
  });
}
// do it every hour
setInterval(syncEverythingToGlobalDB, 3600000);
Notice I recommend using pure CouchDB for this because then you're using CouchDB's replicator. If you used PouchDB's replicator, then you'd have a lot of Node EventEmitters in one process and it would probably be inefficient; CouchDB is much better at handling many simultaneous replications. Edit: heck, you could just use CouchDB's continuous replication instead of doing it in a setInterval.