An example: Rapportive was a browser plugin that put social information (from FB, LinkedIn, Twitter, etc.) directly in your Gmail.

This was great from a user perspective (puts a picture with an email so you recognize that this is 'Bill' who you met at the hackerspace even though his email is [email protected]).

However it was NOT so great for Gmail:

1. Rapportive overwrote the section of gmail where they display ads.

2. It further entrenched Google's social competitors.

There is no way that Google was going to launch a set of Rapportive equivalent features in Gmail. However, Rapportive was beloved by users and exited to LinkedIn.