Bitcoin didn't need to advertise (though it makes me wonder if anyone did make advertisements for it in its early day). It was a movement to make change to money. Forward-thinking people who understood problems with money, or those who simply wanted an alternative to traditional means, had been participating in this topic long before Bitcoin (since the 90's in fact) and there were already predecessors trying to do what Bitcoin did. Satoshi made it happen. He created Bitcoin and this forum, which bought people together to support its development. People powered the network, yes word of mouth would have factored into it (as does with anything good), but also utility, use-case, the fact that Bitcoin was real and could be used, showing current and future promise, all being enough to bring new people onto the network and into the communities.

At that time, the exchange of information around the world was much like it is now, although it has been updated a little over time. It can be said that at that time we were quite close to modern society.

Bitcoin is a store of value, but in the beginning it had no value. The historical event that happened with Bitcoin in 2010 was that 10,000 Bitcoins were paid for just two pizzas and maybe at that time it's worth was only $25. It is really surprising that in just a few years, the price of one Bitcoin is now $100k for1 BTC.

Then the promotion of Bitcoin started through various media and online community worldwide. The forum through which you asked the question initially played a major role in the adoption of Bitcoin from that time. This community is still working to spread Bitcoin worldwide and revolutionize it.

Word of mouth is important part of Bitcoin exposure to communities and its adoption with time.

By reading Bitcoin history, you can know about more events in Bitcoin history, and can see some advertisements in the past, officially or unofficially, depends on how you see these past efforts.

Some events like Bitcoin pizza, honey pot, Silk Road, Mt.Gox, and other funny sales in early years.

• Raw Thai Honey for Bitcoins, another classic just after Bitcoin Pizza trade!

in 2009 it was discussed in the tech community of a small group called cypherpunks who wanted to design a new digital money, and discussed it to their friends and colleagues(mainly all coders)

in 2010 the gossip stretched from the friends and colleagues of cypherpunks to those generally interested in a new money system, and coders who wanted a new project to intrigue them
this started the gossip of its libertarian principles which started gossip in the libertarian communities.. those learning about it started creating services. which lead to other services,
EG a few coders who also happen to have libertarian principles and noticed bitcoin offered some libertarian features, discussion and announced bitcoin on their other libertarian community forums and platforms and groups, spreading the word outside the coding community

by 2011 there were exchanges and services. including silk road. which is where the dark web of hackers and drug dealers also learned about bitcoin. also legitimate finance guys started learning about bitcoin via things like mtgox and other services. also friends of friends of friends of the original bunch started spreading the word for general use.. though the general public use was low amount of adoption. bitcoin was still seen as a code project, a digital money experiment and a libertarian principal type of money

then mass media started talking about bitcoin and talked negatively about it due to silk road, which is where the general public started hearing about it, yep originally general public news(advertising/mass public announcement) of bitcoin came about in ~2011 and it came with negative connotations

Several factors contributed to Bitcoin popularity which range from Bitcoin potential for high returns, volatility, fluctuation, investment speculations, media coverages and public awareness, influencer endorsement  and growing adoption by merchants with the development of some supporting infrastructure such as wallets, exchange and payment processors are some of the major factors that facilitate Bitcoin popularity.

https://www.quora.com/What-led-to-the-popularity-of-Bitcoin-and-why-did-people-invest-in-it-before-it-became-mainstream-Was-there-any-insider-knowledge-about-the-future-of-cryptocurrencies

Even in 2010 i honestly dont recall on google searches bitcoin placing tiny ads on the sides of google searches, i honestly can say this!  So bitcoin right from the beginning never advertised?  Crazy!

I have the slight impression to have heard about Bitcoin back in 2011 through a virtual media on the internet, but I'm not sure how it was exactly, because I didn't pay attention to the news. Probably that must have happened to many other people as well between those past years.

The fact is that many of us have heard about Bitcoin at some point, but we were negligent to not develop further interest for it on that moment. If we weren't negligent, all we would have to do was to create a BTC address in an online wallet service, access a faucet and claim 5 Bitcoins at once. And then we would already have enough money for our retirement decades later...

It was easy like that, but we were more focused in worthless things, which didn't add anything to our lives. Anyway, it's not time to complain about anything. At least we are here now as Bitcoin adopters, trying to thrive somehow in this chaotic world.

it was all organic if someone had sent you information about bitcoin you know that it was a real genuine bitcoin enthusiast there were a lot of methods that bitcoin was “advertised” but nothing in traditional sense like tv or social media

this forum was a big part of it of course and i heard that bitcoin was being given for free at some point back then but there was no bitcoin advertisement in traditional media i don’t think bitcoin was reported in newspapers or anything like that

I don't understand the question. Why did 50 million people or more in the U.S. alone need to know about it? Those that are interested in technology will pay attention to what's going on. Those that aren't interested, won't.

It takes time for a population to learn about a new thing. It always has. It always will.