I am trying to write a 3-5 minute speech that will persuade millennials to take advantage of social media networks (Spotify, Instagram, Snapchat) to keep up with the current political topics and encourage other millennials to vote in the upcoming election, since their vote is so important. I am having trouble because the topic is so broad and there is so much information to research. I am having difficulty narrowing it down. What would be an example of a narrowed down topic that would still accomplish my goal?

Your desire to narrow this topic is a good one, because right now you're trying to do several different things. All of them are substantial, and putting them together is likely to overwhelm a speech this short.
Just for clarity, you are trying to do several different things:
Persuade millennials to vote.
Convince millennials their vote is important.
Persuade millennials to use social media networks to keep up with politics.
Persuade millennials to use social media networks to get others to vote.
That's too much to cover in one brief talk, so, as you thought, you want to narrow down the idea.
To start, there are two general ways to cut it down:
Select the most important item or items and eliminate the others.
Identify the most important item and make everything else a stepping stone to that goal.
Personally, I'd combine the two. I'd try to persuade millennials to vote (because their vote is so important), and to enlist others to vote in all ways possible, including by talking about it on social media.
I'd try to persuade them to vote by highlighting the most important issues for this audience. That would be what I'd research first: what millennials care about politically (money, gender equality, climate change, etc.).
Your visual aids could be examples of using social media networks to learn about these topics. This will demonstrate why millennials should use social media networks to do the same.
If you still need to narrow it further, select one example for each of three social networks. That will structure your speech.
https://www.theatlantic.com/politics/archive/2014/05/everything-you-need-to-know-about-millennials-political-views/371053/