As long as you're comfortable with letting other people know that your views on some social issue was different in the past, you can change as much as you want and still have all of your different selves tied to a single online identity. Companies that track your preferences can also learn to give more weight to your recent activities, profile you based on your background, etc. They will never be able to capture exactly who you are at any given moment, but they can be reasonably accurate on a timescale of months. If there's enough demand, they could even produce a timeline showing your changing preferences over several years or even decades. For example, it would be really interesting to chart a person's movement through the Political Compass [1] over his lifetime, instead of just representing him as a point on the graph.
On the other hand, if you want to use different personas simultaneously, the identity system breaks down immediately. Advertisers, of course, hate that. It might be possible to target ads for someone who changed from X to Y over a period of 5 years. But how do you target ads for someone who pretends to be several contradictory things at the same time?