Where does <3 come from?
Before Unicode characters, especially emoji, were widely available on phone keyboards and the internet, users combined the mathematical less-than sign, <, and the numeral 3 to depict the heart symbol, oriented on its side.
The <3 symbol joins other emoticons, like the smiley :), as popular way to convey emotion and mark tone in early email, text messaging, instant messaging, and online forums beginning in the 1980s.
Once Unicode 6.0 supported emoji in 2010, people widely began using the red heart emoji (❤️) or any of its expressive variants instead of the <3.
Who uses <3?
Since the 2010s, as noted, emoji have become so widespread that many are more likely to type a heart-based emoji than its predecessor <3 (or ♡).
<3 and heart emoji can be used to express love or affection for a person or fondness for some event or content. Like the heart symbol, <3 can stand for the word love or heart, e.g., I <3 New York.
<3 still maintains currency and popularity, however, especially on a computer keyboards, as typing <3 can be faster than retrieving an emoji.
Many people intentionally use <3 to call to mind the earlier days of the internet or demonstrate familiarity with its culture. Often appearing in the phrase i <3 u, the <3 can convey a cute, more innocent, or more earnest tone than various heart emoji.
In colloquial speech and occasionally in writing, <3 is pronounced or represented as less than three. For instance, someone might type less than three if fawning over a picture of an adorable dog. In so doing, users again conjure up the internet of the 1980–90s and signal an "in-the-know, old school" credibility with the conventions and slang of the era.
To add extra emphasis to the <3 emoticon, users tend to add extra 3's instead of typing the entire emoticon, e.g., <33333 instead of <3<3<3<3<3.
<3 is sometimes incorporated into kaomoji, a popular Japanese style of emoticons.
We <3 this colorful and modern Texas wedding!
@theknot, January 2016
Find you someone who looks at you the way Jonah Hill looks at Emma Stone! <33333
@emmabung, January 2017
Pouting in anime. "Oh, so cute (^-^)<3!" Pouting in real life. "What the fuck are you doing mate cut that shit out."
@InvertMouse, January 2016