Steam

Steam presences might be the simplest to fetch simply because they provide a public API that lets anyone fetch anyone's presence. It has an extremely generous rate limit and even supports fetching users in batches of up to 100!

The endpoint documentation can be found here.

There are two main drawbacks to this:

• Steam doesn't send you status updates, you need to regularly query the API
• Steam doesn't send the rich presence through this endpoint (for example "Competitive - Dust II [8:7]" for CS:GO)

These two issues would be solved by using the friends network that the Steam desktop client uses, but I haven't looked into it yet.