OpenSocial looks good. The problem will arise if your website becomes big. You will have a nightmare trying to support the traffic because most of these CMS come with a lot of bloat as they are meant to work in all circumstances and are hence not specific.
My Advice: If yours will be a niche social network e.g for accountants or such, use a CMS and extend it a bit to your specifications. But if you expect this to explode, you can't afford to use a CMS. Everybody will tell you that the hardest thing to do is to change the codebase especially when in a crisis. That's how users leave.