It's a business with a very strong network effects: you need both sides of the market because without drivers, users are not interested and without users drivers are not interested.
Uber seems to have a big lead over competition here, is growing like crazy and they raised insane amounts of capital so that they can grow faster than competition.
But even without network effects, I don't see that as an easy to replicate business: * you need a great iOS app * you need a great Android app * you need a backed that is both fast and has rock solid 24/7 reliability. it can't be like Twitter in early days * you need to become a payment processor that interjects itself between users and drivers, which I'm sure comes with its own set of legal and logistic challenges * and you need to do it internationally * and you need a massive operation team whose job is to onboard new drivers (recruit, train, supervise) * you need a legal team because every state can have different rules * and you need them also internationally
I really don't see any of the above as easy to pull off and to do it all well, that requires both technical and operational excellence that few have.