First up, blogs. They're the easy thing to kill, and FB and G+ are hard at work on it right now. Let's face it, once this post goes up, I'm going to be cross-posting it to G+ right away. I see more responses, more quickly, and am more easily able to respond. It feels more natural. I can easily see this happening to blogs in general.
Make no mistake, some form of blog will hang around for a long time. People still write articles for sites, and some of those sites provide a space for someone on staff to have an individual page. That will be their blog. But the personal blog is, I think, going to have a very hard time surviving. As one piece of anecdotal evidence, I find myself posting to G+ much more than I do here. I can't help but think that others are going to find themselves wondering if their time is being wasted maintaining personal blogs, and just skip straight to one of the social networks. There's less effort for them to do so, after all.
The second topic, though, forums? That will be the interesting one, I think. Forums will be harder to move, since they are a collection of people talking about specific topics. For instance, XDA Developer forums have quite a few major forums, and those even have sub forums. In a single stream, such as that provided by social networks, these things will be buried and missed, making the user miss quite a few topics they would be very interested to follow. Add in that forums provide a ready made situation to meet people with similar interests, and they can be a very tough thing to replace by social networks.
Even with that, though, I think it's something being worked on FB and Google right now. Once they can figure out how to get you connected to other people who share your interests, and then provide a conversation model that gives you topics such as what can be provided by a forum, most forum software will die. Again, the ease of talking on the social network will outweigh the "difficulty" of visiting the remote site.
Forums will have to provide something that social networks cannot. Right now, the only things seem to be having forums dedicated to specific topics (instead of one massive stream), and the ability to be anonymous (especially useful when venting about work related issues).
I'm not sure what the future holds, but I am pretty sure that the social networks will only grow in importance and capability. It looks to be interesting. I just hope the forums can figure out how to provide something the social networks cannot before it's too late.