I don't think they can. Facebook is a good product. You might not like it or whatever but technically and objectively speaking it is very well made with active development that is constantly adding features and tweaking the design and generally staying on top of things. People aren't just going to leave for some ideological reason if the product is servicing their needs. (Consider the crazy ideologically backed brouhaha that happened on Reddit recently, and yet here we all still are...)
Hangouts was basically Google+'s best feature but even that's been basically matched by FB messenger and other services. So it's just a case of too little, too late. We talk about innovation and staying ahead of the curve in tech. But Google hasn't been able to do that at all with social, by the time they launched Google+ people had already moved on from profile-centric social networking sites and along comes Google trying to offer a new one completely missing the point. Meanwhile, in the time since G+ has been around we've seen enormous success of several new social networks that have sprung up around FB and filled niches. One thing they've all had in common has been a focus on being fun and engaging right out the bat where Google+ has been trying to corral people into its network. There's just nothing compelling there, nothing fun, nothing lively.
And all that is aside from the fact that their service isn't better than FB. It just isn't. At best it had a few novel features at launch that FB didn't have, but by today FB runs laps around it in damn near every way shape and form and as per above, FB actually has a very aggressive and proactive development cycle.
This isn't when Myspace users migrated to FB where Myspace was a horribly built website that fell apart, suffered downtime, was inundated with spam and applets that could crash your browser and was just generally a piece of shit.
For Alphabet to have any chance in hell of unseating FB with any product they'd have to have something that came out of left field completely and was radically different and provided something fun and interesting that people wanted to engage with. and even then people would still largely keep their FB accounts.