PHP isn’t going anywhere, and I don’t think that Facebook is the reason for that. It always helps to have some large brands behind a language/platform that could prove the stress testing factor, but it’s not a necessity.
In fact, often the largest and most massive platforms are rarely used statistically speaking. They just require a large number of people working together for a long time on a single project. PHP allows you to spin off a site in a few hours in any of the popular CMS or even frameworks.
While WordPress powers 32% of the web (stats from last week) other popular platforms built in PHP existed prior to that, too. Think of other CMS (like Joomla! or Drupal), photo management engines (Gallery), forums (phpBB, IP.Board, vBulletin), Wiki/docs portal (MediaWiki).
When it comes to web applications, the LAMP stack is the most widespread one. PHP is widely supported, fast, less resource-intensive and cost-effective for hosting vendors who can afford to provide low-cost hosting plans. That reduces the entry investment for small clients and allows for various affordable hosted solutions, too. Try hosting a small site on a .NET or Java web host and compare the resources and costs.
That said, the reasons for the very existence of PHP are a lot, but it got some traction many years ago and the community behind it is enormous. It’s currently used by about 82% or so of the Web. It’s supported practically everywhere and it’s both applicable for small blogs and websites or massive web platforms.
In spite of the common opinion of other developers for the code conventions or the broad PHP community, it isn’t going anywhere and a new successor would require a massive change in numerous industries in order to be just as accessible and affordable in order to make such a difference.