Google and Facebook are not immortal beings infallible of mistakes. These two entities are companies comprised of people. At whatever level you look at inside these two organisations, you’ll see people making decisions. Whenever there are people involved there are bound to be some inefficiencies and mistakes made on the way. These inefficiencies and mistakes can be masked however because of the size of the organisation and the actual impact of a single decision made. Nonetheless a series of compounding “mistakes” could lead to these companies being open to other competitors.

A strength of these two companies lies in the amount of talented individuals in these organisations. Google and Facebook both have a reputation to hire the best and brightest. If this is the case, then we can assume they’d have the raw ability to make right decisions. (debatable point) Given enough time and resources these people could do almost anything. This can be illustrated by the number of moonshots being attempted at Google.

Another point of strength of these large companies is the ability to diversify. Diversification comes in handy when you’re in an ever changing world like we are in. Diversification comes in the form of external investments into potentially game changing industries and technologies (AR, BioTech, etc). Alternatively, change could come from within the organisations, albeit a little harder and riskier,

These are a few of the strengths of large companies like Google and Facebook. Despite these strengths new opportunities and new competitors will still arise. Google is a search engine, but it has moved into email and other markets. Google had existed before Facebook, and Google had more resources than Facebook when it got started. Why didn’t Google takeover social networking instead?

The OP posed the question about if Google will always be the default search engine it has acted as for the last two decades. In my opinion, this can change. With the advent of Amazon’s Echo and Siri, more and more searches are being conducted via these platforms. Both of these platforms use the Bing search engine. [1][2] If Bing did a better job [3], then it’s conceivable that more marketshare can be had by Bing. As human computer interface progresses, the act of visiting Google.com will become antiquated. This leads to opportunities for other search engines to gain adoption more or less transparently.

Facebook is a social network we access via our web browser and mobile device. If VR becomes what is promised and more widely adopted, then social networking as a category will become redefined. Facebook is at an advantage because of the Oculus acquisition, but the VR space is still so young; There are no real experts in VR.

TLDR; A series of “mistakes" made by these people comprised organisations could lead to true competitors. Strengths, like talented people and diversification, could counteract any risk to competitors. New platforms like Echo, Siri, and Cortana abstracts the use of the Bing Search Engine. VR could redefine social networking which opens more opportunities.

[1] https://www.quora.com/Can-I-change-Siris-search-engine
[2] https://www.reddit.com/r/amazon/comments/2lsg9n/amazon_echo_...
[3] http://thenextweb.com/gadgets/2015/07/08/alexa-y-u-no-answer...