Up until now there hasn't really been laws addressing social media which falls outside of what is considered "Rundfunk" (basically television and radio mainly), but over the last year or two a lot of people have started demanding to apply the existing laws to social media as well. I think at the moment they're only required to label paid political ads.

Culturally though it has a greatly diminished role. If I remember correctly, during the last European election the largest parties spent about 500k on social media ads. We're still overwhelmingly dominated by traditional media. Angela Merkel has no social media presence, and if I had to guess even the most popular politicians maybe have a few ten or maybe a hundred thousand followers on Twitter.

You’re saying they spent 500k over a few weeks? How is that even possible?

You just have legally enforced spending limits.

Someone running for parliament in the UK, for example, is allowed to spend about $0.26 per voter † in an average constituency (of 72200 voters).

It's certainly not a perfect system, but it does mean if you earn £100k it's very easy to become an MP's largest donor.

† (The rules are more complicated than that, of course - there's separate spending by the national party, one free mailing by the royal mail, rules for tracking the equivalent value of volunteered professional services, and so on...)