Here is the question: Are they committed?
In other words, are they committed to their seeming racism, their seeming sexism, their seeming homophobia, etc. Also, are they committed to being right, to having the last word, to insulting you, to projecting their stuff onto others (you), etc.
I have found that there are lots of people who make what may appear to be bigoted, insensitive, mean-spirited comments, especially on the internet, where they are assured at least some degree of anonymity. However, at the same time, they are not really *committed* to reflecting those things. In some ways, not being committed in the way I am using that word here is the same thing as being open. However, it is not entirely the same as being open. Similarly there are those who make make some harsh comments here and there but they are not *committed* to being harsh.
Here is what I have also found, if you determine the person to be committed, in the way I'm using that word here, it is probably best to run, not walk away from the discourse ASAP, unless you are similarly committed and/or are a glutton for punishment.
Here are some indicators that a person is most likely "committed in the way I'm speaking of here:" If their first interaction with you on a facebook or blog thread involves them calling you one or more derogatory names; if they quickly move into belittling language; if they point blank say you somehow have no business being in the discourse in the first place (this one means you are being supremely otherized and the person otherizing you knows enough about the owner of the page to know that soon others will join in also stating that you don't belong in the discourse and you will be well on your way to being ostracized); if a posse of people on the thread quickly assembles around the most insulting, mean spirited, foul mouthed and projecting person on the thread and you have been a target of this person; if someone is repeatedly dismissive of you on the thread; if a person actively enlists others to join in denigrating you on the thread; if a person makes the types of comments directed at you that the average person would deem really rude and they are completely unapologetic about it and/or openly revels in it...
Also, the supreme rule of engagement on the internet: You do not have to attend every argument you are invited to.