In which case there is no need to nationalise, because by definition if another competitor can be created then the government can create one itself.

All nationalising would do is give the government a leg-up in having an existing user base. In practice government inertia would lead to the service atrophying and becoming outdated and overburdened with bureaucratic controls and costs. Page & Brin, Bezos and Zuckerberg (or someone new) would each raise capital, start their own competing services and in a few years time we'd be back where we started.

This whole idea is so utterly misconceived and pointless it blows my mind how many people here are trying to justify or defend it. Unless you actively regulated to prohibit competing private services, it would be doomed from he start and if such regulation was enforced - literally banning other search engines including Bing, banning other social networks, banning any other online retail businesses, the lurch towards dystopianism would create a very different world to the one we live in. Even China hadn't come even close to anything like that.