More than the number of users, Twitter’s future rests on the amount of engagement it can show those users having with content from advertisers — and the data it can provide to those advertisers and partners such as TV networks.
I don’t like Twitter, but I think it is a stronger business than Facebook. That said, I don’t see how their business (the part that makes money, ads) is sustainable while they allow third party services to use the API. The most “engaged” users are the ones actively trying to avoid ads. That’s the biggest threat to Twitter making money.