Search Results: adding friend on facebook

I'm sure this isn't what you're supposed to do, but I've requested some girls on Facebook that I think are cute and sometimes I get denied, most of the time they don't accept or deny. But not many accept.
I had one girl accept last year, and we dated for 5 months, but that was the only time I got that to work at all.
But I did this recently and these three girls all accepted my request within the last day. We have no mutual friends, and they don't know each other. They also don't have an excessive amount of friends. All 3 have 2-300 ish. But I'm not sure about trying to. talk to them yet. They have in their profile they are single.
Why did they add me?

I had a friend through highschool I really liked, but she wasn't ever a good friend. She'd only hang out with me just to try to get me to help her boyfriend with car audio or something of the sort.
After she disappeared during a hangout, we didn't talk for two years, until she added me on Facebook. She would comment on a lot of my posts and tag me in a lot of stuff, but then she got a boyfriend.
A year later, I found her on tinder and I asked her out. She was reluctant because we were friends before, and the date was ok. But nothing great. She wouldn't bluntly say anything about it and we were gonna try again before she called it off and was very insulting.
She messaged me a few weeks later like nothing happened, but it was a brief conversation. After which I blocked her on Facebook, Instagram and her phone.
It has now been a year and she texted me with a new number asking for some favors with her car and and issue in her house. She even texted a few hours later saying who she was.
I immediately deleted the message and am frustrated about it. I feel she's trying to use me, but someone suggested she's reconsidered and wants to try dating again.
Why did she? And will she do it again?

When you're using Facebook and you add someone you want to catch up with or try to get to know, and they never interact with you and they ignore your messages. Is it justified to unfriend them?
If it's to try to be friends, catch up or date them.