What I really need someone to tell me is this: do I text, or do I facebook message?

Is there an unspoken of qualifier or criterion that must be taken into consideration when trying to transpose information to someone who is both your fb friend and in your list of contacts?
Here’s what I’ve been operating on. Please tell me if I’m violating a social media faux pax. 
1. Time immediacy. Even though most of the social sphere is on smartphone time, at least 50% of my family isn’t, so if I need a timely response, it goes text. 
2. Privacy. Those thoughts that I don’t want to disclose to other people go via text. Even though no one else can read a fb message, for some reason the text just seems safer. So a snippy text about a co-worker’s habits or a mutual friend’s dressing habits frequently go the way of text. As if there’s a FB message screener but Verizon would never think to look in upon such things. Or I have an underlying sense that the VZW troll keeps his mouth shut. 
3. Relational proximity. Though I might have them in my phone book because I called them 2 years ago, that doesn’t mean a random text is appropriate. Perhaps Zuckerburg had a greater sense of social hierarchies than we thought when he created the FB message. 
4. Content. It seems that nearly all FB messages require response whereas texts do not. So there must be an underlying sense of seriousness. This does not mean I respond to my FB messages because anyone who has sent me 2 or more knows that I do not. This is a laziness issue, not to be confused with a lack of social media clarity.  
What did I miss? 
Visit me elsewhere: