So you're not interested in a fixed solution but want to program it yourself, do I get that correctly? If not: Go with a fixed solution. This will be a lot of programming effort, and whatever you want to do afterwards, doing it in another framework than you intended will be a much smaller problem.
But if you're actually interested in the programming experience, and you haven't found any tutorials googling for, say "messaging python tutorial", then that's because these are large-scale projects,- if you describe a project of this size, you're so many miles above actual lines of code that the concrete programming language almost doesn't matter (or at least you don't get stuck with the details). So you need to break these things down into smaller components.
For example, the friend/follow function: How to insert stuff into a table with a user id, how to keep a table of follow-relations, how to query for a user all texts from people she's following (of course there's also some infrastructural issues if you hit >100.000 people, but you get the idea ;). Then you can ask yourself, which is the part of this which I don't know how to do in Python? If your problem, on the other hand, is breaking down the problems into these subproblems, you need to start looking for help on that, but that's probably not language specific (so you might just want to start googling for "architecture friend feed" or whatever). Also, you could ask that here (beware, each bullet point makes for a huge question in itself ;). Finally, you could get into the Pinax code (don't know it but I assume it's open source) and see how they're doing it. You could try porting some of their stuff to Pylons, for example, so you don't have to reinvent their wheel, learn how they do it, end up in the framework you wanted and maybe even create something reusable by others.
sorry for tl;dr, that's because I don't have a concrete URL to point you to!
