Once this is widely accepted (and I'm sure there are many that would argue with me), the thing that will separate these types of services won't be whether they stay up (ala Twitter), but how fast your subscription messages are updated. Some services might be smaller or offer more features but not update as quickly whereas others will pride themselves on being as close to real-time as possible. The key is that it's all about messaging, not publishing. (Oh, and this also facilitates federation as well, but that's another topic).