None of the answers here seem to actually answer the OP question. So here is my attempt:
Many social networks actually provide guidelines on how to use their logos, sharing icons and other links. Some are more flexible than others on that subject.
Of course, in most cases, they won't fit your design really well and you might want to customize them with your own theme/website colors.
"Do this at your own risks" is probably the best answer you will ever get.
I am not a lawyer (so this is no legal advice!) but from what I could read in different articles, some websites that were selling icon sets with customized social network logos were asked to remove them from their products list.
There is little chance that Facebook, Twitter or any other social network company will sue you (although they probably could) for having customized their icons/logos for your personal blog or website, but if you want to cover yourself, you probably want to consider following their guidelines...
Here are a few links to social network brands guidelines:
- Facebook: https://www.facebookbrand.com/
- Twitter: https://about.twitter.com/company/brand-assets
- Google+: https://developers.google.com/+/branding-guidelines
- StumbleUpon: http://www.stumbleupon.com/press/stumbleupon-image-use-guidelines
- Delicious: http://delicious.com/branding
- LinkedIn: https://brand.linkedin.com/
- Tumblr: https://www.tumblr.com/logo
- Reddit: https://www.reddit.com/about/alien/
- etc.
You can search the web for "[brandname] brand guidelines" to find these guidelines for almost every social network.
Some source for the above information if you want to read more:
- http://www.evolvecreativegroup.com/the-secretly-strict-brand-guidelines-of-social-media/
- https://www.quora.com/Internet-Law-Is-it-legal-to-alter-Facebook-and-Twitter-icons-to-match-the-rest-of-your-website
- https://webmasters.stackexchange.com/questions/54499/putting-altered-social-media-logo-icons-on-my-website-can-i-get-sued
- http://www.kgshultz.com/using-modified-social-media-icons/
- https://www.nosegraze.com/allowed-use-modified-social-media-icons/
Hope this helps.
