It looks like you already have had a conversation with her about this, and it didn't help. If she continues to behave in a way that hurts you after telling her how it hurts you, you really should reconsider this relationship. However, sometimes we think we've been very clear and/or very kind, when we weren't. Our choice of words and tone heavily influence the other's response and understanding of the situation.
Here are some tips I've found useful in the past, for difficult conversations like this one. The outcome is never guaranteed, since you can't control the other person's reaction, but the things I'm going to list have often lead to more meaningful and positive conversations.
Do some introspection
There are a number of reasons why your girlfriend would do this. Some have nothing to do with you (did she have an ex who crossed her boundaries, leaving her with some issues around that; or this is just how she handles fights). Some could. For example, do you act inappropriately on those social media platforms?
Take some time also to figure out what you need for this relationship to work. Do you need her to stop this behavior? Or would an explanation help mitigate your hurt feelings? Would a warning or a message telling you she wants to be left alone for the next few days work? The more clearly you know what you need, the better you know what to ask, and the higher the chance you finally get what you need. The goal here is still to have a discussion with your girlfriend and find a solution together, but you need to know your own boundaries and needs first for that to work best.
Last thing to ask yourself: is this a deal breaker?
Tone is (almost) everything
Pick a time when you're not upset. Don't do this right after she blocks or unblocks you, that's when the your hurt will be the freshest and you'll be more quick to get mad. I wanted to say to do this face to face (or over Skype), but some people take things better over e-mail or chat (and you do have a long distance relationship), so pick the medium where you communicate best with her.
Your tone and facial expressions really matter here. Coming off as frustrated or aggressive will make the other person feel attacked, and they'll become defensive and stop listening. So try to have a kind, warm tone, like you're having a weird issue and need her help (instead of demanding things).
Keep the focus on your feelings
Keep the focus on your own feelings and on how her actions affect you, instead of flinging accusations at her. Try to use as much I statements as possible and use passive language (no "you"). This could look something like:
I feel hurt when you block me on social media. I know we've already talked about this, but it keeps happening and when it happens, I feel awful. I need either for this to stop or to understand what's happening, because I don't want to be hurt like that anymore. And if there's something I'm doing that is hurting you, I want to know too. I just want both of us to feel happy and safe in this relationship.
If she wants to stop the conversation or gives you a short answer, stay calm and warm and keep the conversation on track. Give her time to respond to you. Be patient. This is apparently a sensitive subject for her, so give her a safe space to open up about it.
Don't let her get away with it
I'm glad you're not blocking me anymore, but I'm hurt this happened again and would like to know why. It's getting really hard for me to move on from this and act like everything is OK when it isn't.
If she does it again, you don't have to just brush it off. Send her a short message telling her that even if she is over whatever reason that made her block you, you're not over this. How hard a stance you take depends on how much a deal breaker this is.
Again, if someone continues to do something that hurts you, they know it, and offer no explanation or apology, it's time to question that relationship. But more than that, if she's even unwilling to have a reasonable discussion about this, it tells a lot about how disagreements will be resolved in the future, so I'd also ask myself if I'd be OK being with someone who's not willing to address issues and work toward a resolution.