As with other similar modern situations, one has to refer to the Four Great Standards in the Vinaya to find out whether something fits in with what is allowed or with something which is not allowed by the Buddha. Yes, monks can take and make use of some basic necessities which have no individual owner, such as drinking water or firewood or cloth which has been discarded/left over at a shop. In the same way, we could say that the internet services nowadays are offered freely to anyone who signs up, but there is a catch: most of them will be financed by displaying ads, in the hope of attracting potential customers. However, there is no actual contract between the company and the user promising to buy things from them. So a monk is no different from a layperson in such a situation. More fundamentally, though, the internet was initially created mainly for the purpose of disseminating knowledge and academic discussion, which is something that monks can also contribute to.
Kiṃkusalagavesī anuttaraṃ santivarapadaṃ pariyesamāno... (MN 26)
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