I think greeting 100 people in a day would be a monumental task for anyone and an impractical notion for most of us not making a YouTube video. But, the point is a good and valid one.
If you think about it, no one would ever smile or greet another if one person wasn't assertive enough to make the initiating move. It does help when there are fewer people around, be it a small town sidewalk, empty store aisle, cold day, or remote area. But even in a busier environment, the difference between engaging with people or being isolated starts with looking at them.
A look, more often than not, is a social invitation. It's better than nothing, since it opens up your space or willingness to engage with another person. For those who recognize their need for social interaction but are too shy to be assertive, I'd encourage them to at least look at people when passing by, standing in line, or sharing an elevator - particularly in an uncrowded setting. And don't we do the same as single guys or girls who see someone we like? We show interest by looking, thus encouraging a response.
In my experience, a look accompanied by a smile, nod, or greeting almost always encourages a positive response - like 9 out of 10 times in any situation where the contact is close. Any decent person feels compelled to respond favorably to a warm action of another directed towards them, be it opening the door for them, letting them go ahead in line, or offering a greeting of some sort. The high odds of success, unlike asking someone out on a date, justify the risk of being assertive; the question then comes down to whether you care or want to.
I'd propose that proactively engaging with strangers, even for just 2 seconds, is a win-win deal that's worth pursuing. It not only makes the world a better, more friendly, loving, safe place, but it gives self gratification to both parties - the one who initiates and the one who responds. And inevitably over the course of years doing this, one's kind gesture will hit someone at a critical moment when they desperately need it.
Initiating a friendly contact with at least one person every time we go out is not an unreasonable goal. And the more we do that, the harder it'll be to settle for just one - because it feels good.