One reason is clearly because the number of Internet users keeps increasing but I'm looking for deeper answers.
The core underlying factor for this is that platforms and behaviors took hold which did two crucial things for commerce, communication, and entertainment driven companies. The first was provide a graph to leverage for quick scale, Zynga on Facebook being the best example. Many others were able to leverage the social graph to quickly grow and you could include Pinterest as a benefactor here, as their core group of early users were already custom to Facebook (and other large niche networks like Etsy).
The second was that the social and mobile web on-boarded a huge swath of new users who were increasingly comfortable doing things online that they previously did offline. Purchases, communication, and self-expression really began to hit a stride because Facebook, Tumblr, and eventually Twitter attracted large audiences that you might consider "mainstream", and the companies that served those markets benefited greatly. As an example, GroupOn delivered deals via email at a time when nearly everyone had moved to web-based email solutions (and had personal emails - not just one for work). Ebay and Amazon had gotten people comfortable with purchasing online, and Groupon pitched them discounts and ease of redemption.
The big, big successes are usually about timing, catching trends that are inevitable. Twitter's an example of nailing both mobile-first and the media's inevitable transition to the web - celebrities, athletes, and big brands were the first core success stories that showed the platforms true potential. It was a product built for right now, launched seven years ago.
For new entrepreneurs, the real win is that not only are more platforms available today than ever before, but more "building blocks" exist today than ever before. You don't have to reinvent the wheel for a lot of the moving parts of a new business, and you have an almost overwhelming number of choices when it comes to finding new ways to reach an interested market.
Anyone interested in thinking about how to reach their intended market is more than welcome to reach out to me on Clarity.