Two of the three parties have set up their own Mastodon instances years ago (grüne.social and social.linksfraktion.de respectively).

The Left is also present at @dielinke@mastodon.social

And even high ranking SPD politicians are active on the Fediverse (@eskensaskia@mastodon.social for example)

They are going to be present on closed platforms, because like it or not (and for the record I don’t) that’s where a large part of voters is, but the Fediverse plays an active part in their social media strategies.