In today's social media dominated world, we only see edited versions of reality. Most likely, this is a result of human nature (the tendency for human beings to want to show off/be admired) and the apps themselves. After all, who wants to follow or watch someone who posts pictures of paint drying? As consumers of social media, we only want to see the fun and exciting, not the boring and normal.
Therein lies the issue: we constantly see the positive aspects of others' lives, but fail to see the negative aspects. There are exceptions of course, but as a generalization, this is true. As a result, I'd argue that more people are lonely than you may think.
This was a realization I came to in the past year through various personal experiences. For instance, during a drunken moment, one of my friends revealed to me that he felt very lonely and didn't actually have many "close" friends. This came as a huge surprise since this guy is one of the more extroverted, social, and outgoing people I know -- he's not at all someone you'd expect to be lonely, at least based on physical appearance and personality. He was even a member of a frat, so at the very least, you'd think his brothers would be close friends.
Another time, after hosting a party, I was cleaning up my apartment and found a bottle of antidepressants. The antidepressants ended up belonging to one of my friends, who similarly never really displayed overt signs of anxiety/depression. I was shocked.
You can call me naive, but I think this is an incredibly important topic, and one for which it would be especially beneficial to discuss on an anonymous forum like WSO. The point is that you are not alone if you feel lonely. It's perfectly common and normal to occasionally have feelings of loneliness or social anxiety.
We've all had moments when you see a friend's Snapchat story and you wonder, "Why wasn't I invited?" (I guess this only applies to the millennials) Even if you're confident in yourself and know that social media is a facade, it still certainly does not feel good.
Curious to hear everyone's own experiences with loneliness, and anything else related to this topic.
EDIT: I just wanted to add that being alone and being lonely are not the same (this is from some of the comments). You can be happy and alone, and alternatively, you can also be with others and feel lonely.