URL shorteners are something you either use a lot or never. Google launched it's own URL shortening service in 2009 with unique (at the time) features like third-party API access, QR code generation, ability to use easily on mobile.
But Google is retiring this public facing service and replacing it with Firebase Dynamic Links (FDL) accessible by developers only.
This is not surprising since Twitter retired Deck.ly when it acquired TweetDeck.
As part of the process of making TweetDeck more consistent with Twitter.com & Twitter's mobile apps, we're removing deck.ly from our apps
— TweetDeck (@TweetDeck) September 15, 2011
If you have links, Google is giving you until March 30, 2019, to figure out what you are going to do (even though you will lose the ability to create new short links on April 13).
What are the best Goo.gl alternatives?
1 - Bit.ly
The first alternative has to be Bit.ly which is one of the most popular URL shortening services on the internet and one of the oldest. You create an account and then generate short links as required (you can also choose a tag to group your URL).
Bitly allows you to create custom branded short URLs, which is excellent for marketing.
2 - Ow.ly
Hootsuite runs a service called Ow.ly. Ow.ly offers all of the features of Bit.ly but integrates with HootSuite. So if you use Hootsuite to manage your social media presence, this could be the best option for you.
The big difference is that Bit.ly allows you to quickly shorten a link from their main webpage without having to sign-up whereas Ow.ly does not.
3 - rebrandly.com
Many lists include Firebase from Google but I am omitting it since it is only designed for use by developers in apps (not useful for the average Joe). My last recommendation is Rebrandly.com which offers custom URL shorteners. Many large cloud companies are Rebrandly customers (such as Microsoft, Dropbox, etc).
Before you get scared and look away, they offer a free tier that will meet the needs of most users.
Conclusion
A URL shortener is a service that you will rely on for years, and I have presented the companies (services) that look to be the most stable. Remeber that when the service disappears's your links break which could wreak havoc on your social strategy.