The difference is network effects. There are tons and tons of newspapers (over 1000 in the US alone), and one can plausibly start their own. There's a tiny number of social networks and it's extremely difficult to break into that market.
Low barriers + large numbers is at least a nudge in the direction of intellectual diversity.
Internet also offers you large audience instantly. If you really wants your speech to been seen and not censored, you can setup your own website for it.
The US used to have a vibrant newspaper scene with all kinds of papers ranging the spectrum. This was in the 29th century. Since then a handful of giant corporations have dominated the media industry.
Three if you count Perth's local daily, The West Australian, which is owned by Seven West Media. That doesn't really change your point though. But, Australia is a small country (in population terms), and it only has six or so major cities, so I don't think we could expect vastly wider media ownership than we've currently got.
One could argue that we could add should, but we don't. We just think 23 million people can only support two news papers. That way we can claim there isn't a monopoly.
Low barriers + large numbers is at least a nudge in the direction of intellectual diversity.