Whenever you are in a social setting, people tend to talk with others they are interested in in some way, whether it be talking between sexes or within the same sex: either by having common interests, common acquaintances, common experiences, or just plain physical attraction. You probably wouldn't be joking around the same way with some drugged-up loser with meth. scarring and stinks of vomit and urine. So if you define "flirting" as communication to show interest in another person, the mere act of talking to a person is a mild form of flirting. This is even more so when you and the person you are talking to are single (or perceived to be so), young, and of the right age group and sexual orientation. All relationships start out with just being interested in a person and then grows from there as you learn more and more about that person. And as others have said, this works both ways.... I've witnessed when a waitress at a local bar was commenting on the "eye-candy" of the guys setting up their band equipment... When the guy came up to that waitress to order a drink, she immediately got giggly and flirty until he finally had to say that he better get back tot he stage or his wife will begin thinking he was flirting.
I also think it depends on how you are talking... Since you are "joking around" with a guy, you've already established some type of familiarity with the guy; you wouldn't joke around with some random guy on the street (unless you were purposefully flirting with him). So again, the more personal and intimate you get with a person, the more they think you are interested in them. Additionally, some people tend to be more touchy-feely when they are talking; a hand on the other's arm, giving hugs as greeting or when leaving, etc. That physical contact can sometime be construed as being flirty, even if you'd do the same contact with all of your friends (eg. a woman that gives hugs to all of her friends, whether male or female; or the guy that always rests his hand on the person's shoulder, again no matter the sex of the person he's talking to). Finally, there may be non-verbal cues that you are giving off that may be read as being flirty, even if they are innocent. A girl with long hair that flips it back could be giving a non-verbal clue of her interest in the guy she's talking with, or it could just be that her hair was getting in the way.
So I disagree with the statement "just because we girls are friendly doesn't mean we like you", because by the mere fact of you talking with us indicates that there is some type of interest in us, that you like in us in some way. And if a guy is also interested in you, it's probable that he may make further moves just to see how far your interest goes. Eventually you may have to say something that shows you like him as a friend, but do not want to take it any further than that. And the flip-side is true (as well as the other flip-side in which talking with someone of the same sex that happens to be homosexual may try to see how far your interest goes).