You want to help diaspora* by giving a few bucks? Awesome!
Donate to your podmin
The pod administrators are the ones who make the diaspora* network alive. But each node has a cost. To help your podmin keep the server up and running, you can make a donation. If he activates this feature, there should be a "Donate" section in the left column of the main page. You can there choose the donation system you prefer between those made available by you podmin. The diaspora software allows to set paypal, liberapay and bitcoin donations.
Donate to help code to be written
diaspora* is developed by volunteers. You still can make donations to show your interest and encourage them to invest more time in diaspora* development.
The current favourite way to help diaspora* development with money is to use the Bounty Source plateform. This platform allows you to put money directly on tasks (github issues). When a developer writes the code filling the task (fixing the issue) and its work is accepted by the diaspora* core team, he then get the money attached to the issue on bountysource. That means you are now the ones driving the priorities of the diaspora* project! Of course, if you don't have a specific issue in mind, you can give money to the diaspora* team directly and we will attach it ourselves depending on our priorities.
For more info on bounty source, see this blog post.