At what point does something like Twitter become a monopoly or a utility, and at what point does it become anti-competitive for them to block applications from

At what point does something like Twitter become a monopoly or a utility, and at what point does it become anti-competitive for them to block applications from using their service strictly because they compete? Microsoft's 'crimes' of packaging the browser with the OS seem charming now, it's not like they were preventing you from installing competing browsers. I don't know the answer but it certainly seems like as a few of these major social networks grow to dominate our lives the regulators may need to step in eventually.