What do you mean by "most popular."
I'm tired of hearing that "everyone uses..." No, they don't. MySQL is pretty popular with the open-source web-crowd but this is the same crowd that respects the engineering behind PHP. I've encountered plenty of people in that arena who would rather roll their own data-checks and treat the database as barely more than a key-value store than use the capabilities of the database and have to deal with handling exceptions. Bring up transactions, ACID compliance, data-integrity and the like at a PHP users group and you get blank-stares. The get-rich-quick-with-a-cute-kitten-website crowd cares not for such things (as an overgeneralization - there are plenty of high-traffic sites such as Instagram, hi5, Etsy and MyYearbook that run on PostgreSQL).
So where do you find PostgreSQL? Salesforce, National Weather Service, Nippon Telephone and Telegraph, Federal Aviation Administration, Sony Online Entertainment, TD Ameritrade, State of Wisconsin Courts, Afilias, BASF, Flightaware, Skype (a contributor of many PG utilities), Fujitsu, Launchpad (Ubuntu)...
And PostGIS is *the* go-to open-source geospatial database.
I've found the PostgreSQL community to be wonderful with opportunities to contribute at all levels. Answer questions on the mailing-lists, contribute to documentation, help at users-groups, give a talk at a conference. One always welcome contribution is doing testing and submitting results/patches during commitfests - and this gets you more involved with the code.
As to employment, it sounds like you prefer PostgreSQL. As such, PostgreSQL is by definition the most popular database among places you are interested in working. Do what you love.