I'm looking to describe something that people use for no particular reason, other than that other people use it. It is popular because it is popular.
De-facto seemed like a good fit at first, but my friend pointed out that it sounds haughty. Are there any synonyms that effectively communicate the "in practice" concept?
De facto is typically used in various idiomatic expressions. An example is de facto standard which means an industry standard practice without a formal standard behind it (such as an RFC, ANSI, or ISO document). If you are writing for technologists or lawyers this will be immediately understood and taken, not as a haughty expression, but as a well known idiom which distinguishes a de facto something from a formal something or a de jure something.
Other audiences might not understand it. For them you might try generally accepted.
It depends on the context. If you're observing, for example, that many web servers use Apache, it would be entirely appropriate to say, "Apache has become the de facto web server of choice." If you're observing that many teenagers in your town die their hair blue, you probably wouldn't say, "Blue has become the de facto hair color", unless you deliberately wanted to create an odd-sounding phrase. You'd be more likely to say, "The latest fad is to die your hair blue."
Phrases with similar meaning that come to mind -- with varying levels of formality -- are "de facto standard", "accepted practice", "common practice", "trend", "fad", "craze", "pattern", ... probably many more.
I reckon in practice is a pretty good alternative. De facto is best reserved, along with de jure, for specialist legal contexts. Elsewhere its purpose is normally to make an ordinary statement sound more impressive than it really is. See also the question on i.e.
