The only way to flee is to have an alternative. And despite all of the wanna-bes, there are no real quality alternatives.
Or network effects make alternatives less attractive.
Take eBay for example. The network effect makes it such that despite its fees and policies, it remains the #1 site for buy and selling goods.
Sure other sites have started up and are better in many ways, but you see complaints from buyers along the lines of "If I wanted to pay eBay prices, I'd use eBay!" and complaints from sellers of "Buyers are always lowballing me - they refues to pay what I'd get on eBay!". Well, yeah, the network effect is such that buyers KNOW they're using a lesser site so they hope to get bargains (or else they'd just save the effort and use eBay) and sellers are using it hoping to use the lower rates to make more money (but expecting the same prices as eBay). This ends up with auction sites basically dying because sellers want eBay prices despite lower demand, and buyers want cheaper prices because of relative obscurity (again, if they wanted to pay eBay pricing, they'd just use eBay).
Facebook and the others are the same thing - you want me to use your network, but all my friends are on Facebook, so I'd just be making extra work for myself to use your network. You'd better have a compelling reason for me to do twice as much work. (See G+).
It only works if you have the network effect going for you. Something like Amazon doesn't, because I can buy a DVD from them, or a DVD from Walmart.com, so the two are fungible.
eBay is not fungible with any other auction site. Facebook and G+ are not fungible (for most people) - you cannot take a user of one site, transplant them to the other and expect things to go just fine. Amazon and Walmart are, since it doesn't matter where you get your product from - you just use free shipping and pick the one that has the lowest price.