Depending on each social media platform (Facebook, Twitter…), there are different types of sources that you can include on your Wall (e.g. global hashtag search, individual profiles/pages etc.). Here's an example for a source that includes all public Tweets including the hashtag #cats:

Now, this popular hashtag of course will bring in a ton of posts! To refine your source, you can add additional filter keywords and/or hashtag-combinations. The following example will only show posts that include both the hashtag #cats and #play:

Now, you can go even further and add several filter keywords/hashtags. The next example will show posts that include #cats and at least one of the hashtags #play and #cute (in nerd-speak - several filter keywords constitute an OR-constraint):

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