I think all Christians should share the gospel.
(IMO) I don’t think you can call yourself a servant of God if you don’t.
Matthew 28:19
Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit
Romans 1:16
For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek
Romans 10:13-14
For “everyone who calls on the name of the Lord will be saved.” How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching?
We are supposed be ready to preach the Gospel where ever we are, but the modes in which we do that can vary.
The real question seems to be how Christians interact with tumblr, which is a valid question in my opinion. It really depends how you use it. tumblr is like the rest of the world, full of sinful people, therefore, you have to be careful how you interact with it. It is very possible for all the sin on this site to get into your brain and start desensitizing you to evil things. You’re not going to go to hell for that, all sin is forgiven for those who are in Christ, but if it’s harming your walk with Him, then I would definitely take a break.
However, if you are following people that post biblical content and steer away from indecent, profane, or hateful content, then there can be good things on here to enjoy at our discretion. I like seeing people have fun enjoying similar interests as well as Christians encouraging each other with the Gospel.
So long story short, we should preach the Gospel whenever we can, but don’t feel like your a bad Christian if you aren’t doing it all the time. If you can use tumblr in a God-honoring way, use it. If you can’t, don’t.