First name and last name field in online sign up form is the most irritating fallacy that User eXperience designers made. Period.
Assuming that every single person in the world have first name and last name part is just so wrong and ignorance. Even assuming that a person has name that consists of two or more words is not good (an example: my father’s name only consists of one word). Even if they indeed have name that consists of two or more words, making assumption that the last part of their name is their “last name” (or “surname”/”family name” in western cultures) is not correct approach either (an example: Japanese names generally in the form of “family name” followed by “given name” thus the last part of their naming system is not the “last name” in the sense of family name).