I am with you @Snowchoc Just say 'sorry I have made other plans now. You didn't message for several days so I assumed you couldn't make it.' I HATE HATE HATE being fucked around. Just tell me straight, yes or no, and tell me soon please... Within a day if possible! I do have a life, and other friends, and family, and a job! (Although it is flexible WFH hours I am still busy and still have to do a certain amount of hours in a week.) Don't jerk me around for days on end. As you say, it feels like they're waiting for something 'better' to come along.
I have a 'friend' who I have known for quite a long time - 25-ish years, and it's always me who gets in touch with her, and tbh I am sick of it. (Been this way for about 7-8 years.) Before Christmas, (on 1st December,) I messaged her and said 'hey, are you up for meeting for a coffee and a chat soon?' 
She messaged back THREE days later, (on the 4th of Dec,) and said 'I need to look at my diary/work days and let you know.' She works in a job that has differing shifts, and they get their shift rota in advance that they're working the following month. I thought why the F didn't you look first and let me know when you wanted to meet, instead of just messaging me - THREE DAYS after I messaged you - to say 'I'll let you know?' 
I said 'OK cool. I can make it any weekday in December, (w/c 4th December, w/c 11th December, and wc 18th December,) as I am only busy on the weekends and a few of the evenings... Let me know soon if you can.'
(I WFH and have very flexible hours so can pretty much meet any time most of the time!)
It was another FOUR days before she messaged me with a suggested date. So 7 days in total to get a fucking answer!) Normally I would fire her another message if she didn't get back to me within 2-3 days, but I thought 'fuck it, let her contact me. It's always me!' And I regretted saying 'I am free (during the weekdays) all month up to the Christmas weekend,' and wished I had said ' I am free AT THE MOMENT! But may not be for long...'
I wanted to plan a couple of things, like appointments, and things with my DC, DH, and another friend, and because she hasn't answered me, I didn't know what date she wanted to meet, and was worried about double-booking. As I said, I wish I had said 'I am available these days at the moment!'
As I said, I hate these 'I'll let you know' fuckers who piss you about, and make you wait and wait, and keep you hanging. It's so rude. I have decided I am not contacting her again now, and am going to wait for her. If she is interested in maintaining contact, she will contact me. Right now though, I CBA. I met her 19th December, and had a quick coffee and a chat, and haven't heard from her since.